I spent a good deal of time in Japan sniffing around questions like this. Justin hits all the high points -- what I have to say is by way of clarification.
1. Wasabi with soy. Okay, real wasabi, ground from a root not more than an hour or so before you eat it, has an extremely subtle floral flavor that goes away very easily. If you mix it into soy sauce, for example, you'll lose it. If you grind it much before service, it goes away. So if you're using real wasabi, mixing it into soy sauce is sort of a faux pas, not because it's rude but because it indicates that you don't really appreciate what you're eating. It's a faux pas the way, let's say, drinking a light white wine with a heavy cassoulet is a faux pas --- it shows you don't have the palate for this. But if you're using the powder, which rarely has any real wasabi in it --- and even when it does, it makes no difference, because you can't really tell --- you can go ahead and mix it together if you like. Most wasabi in Japan, and almost all wasabi outside Japan, is dry horseradish with green food coloring. You can't really get snooty about how to eat fake wasabi, and people who do clearly don't know what they're eating. Go with what tastes right for you.
2. Soy sauce on the rice. Yes, that's bad. We're talking about nigirizushi here, not makizushi. If you're eating fish (or whatever) on a nigiri (rice ball), you shouldn't get soy on the rice, because it acts like a sponge and sucks the stuff up, and then you can't taste anything but soy. Again, it's not a politeness thing, it's just being an oaf.
3. Ponzu with tuna or whatever. The usual rule is that ponzu is best with oilier fish, because it cuts that oil. But there is no absolute here. It mostly depends on where you are in Japan. The thing is, real ponzu is quite different from the c**p in a bottle. If you want to make your own, you can, but you have to plan ahead. Basically what you do (I can give a recipe if you really want it) is you take a bunch of kombu, the thick dry seaweed you make dashi with, break it up into rough squares, and cover them liberally with citrus juice -- yuzu is most traditional, but lemon or grapefruit are good, and lime rather less so. Leave overnight, covered with plastic, then strain. Then you take very good soy sauce and soak a bunch of bonito flakes in it for about 10 minutes, then strain, and you mix this with the kombu-soaked citrus juice. The result should taste principally of soy sauce, with a definite citrus accent, and there should be a peculiar depth of flavor and smokiness that comes from the kombu and bonito flakes. There's a little more to it, but that's the basics. This sauce is especially good in hot weather, and yes, Virginia, you can serve it with tuna if you like. But it is not very usual to serve it with sushi --- it's more a sashimi thing.
4. Grand tradition and history. Sushi as we know it is a recent invention, from the 19th century somewhere, in what's now Tokyo. It has been hyped endlessly in all kinds of grandiose ways, by Japanese and Westerners alike. You can buy into that or not, as you prefer --- after all, it may be recent, but then again, by that measure so is French cuisine as we know it (think when Escoffier was working, right?). What sushi is not is a Japanese tradition. It's a Kanto tradition, i.e. from the Tokyo region. People in other regions have their own traditions. In Kyoto, for example, you can get very good sushi if you want it, but it's not a big deal. In fact, they have their own types of sushi, which are rather older and taste quite different --- this is the stuff about sushi rice being used as a preservative. Yes, there probably are Tokyo sushi-ya who say prayers before getting really into the work, just as there are Kyoto kaiseki chefs who visit the same temple every day at dawn. But that's not because it's sushi: it's because some culinary traditions in Japan are steeped in an elaborate traditionalism that deliberately sets them crosswise to modernity.
5. Spicy or other mayo. Yes, you see that. It's pretty much low-end cr*p for the kids, but you do see it. Not so much on the spicy, because spicy isn't big in Japan in any form except Korean barbecue, but mayo is huge. If you want to do it, use Miracle Whip, which is sweet and rather more like the stuff you see in Japan. Brits, here is one good place to use "salad cream" -- and there are no others, although I hear it works okay as furniture polish.
In the end, I think the crucial thing to remember about great sushi is that great sushi, in its natural habitat, is bar food. It's a heck of a big step above nachos and peanuts, to be sure, but it's bar food. What you do is, you go to the bar, and you drink and chat with your friends, and as you feel like it you ask for something to eat, and you get it. All that rice helps cushion the blow of the alcohol, and the salt --- well, think peanuts, OK? If that sounds like I'm running it down, think tapas, which is basically bar food too.
My point is that sushi is, or should be, fun. If you're a bunch of gourmets who are really into subtleties, I suggest that you go very light on the fake wasabi, use the very best soy sauce you possibly can, and use as little sauce of any kind as you can. You want to taste the fish. In fact, if I were doing it at home with gourmet friends, I'd skip the rice too --- all sashimi, all the time. Why? Because making sushi rice is a b***h, and it really shows if it's not very good. I'd rather focus on the seafood. Have plain white rice on the side, some homemade Japanese quick-pickles, and a lot of good sake or beer. Get seriously lit, try everything, and roll around groaning after you've eaten way too much protein. If you worry about details, you won't have enough fun, in which case skip it. What's the point of bar food if nobody's having fun?
You really want to do this the fun way, may I suggest a handroll party? Make a bunch of sushi rice, and it doesn't matter much if it's imperfect. Buy a couple packs of the best sushi-nori you can find, and at the last minute toast it gently and quickly over open flame. Lay out plates of seafood, rolled omelet, quick-pickled vegetables, sushi rice, nori, and so on. Put dishes of whatever sauces you feel like nearby. Now announce, "dig in!" What you do is, you pick up a piece of nori, and you mound stuff on it as you like, and you roll it up into a cone. Dip the end into a sauce if you wish, and bite. Dip a different sauce if you like, and bite again. Yes, I know, double-dipping, eek, oh no, we'll all die, get over it and drink some more.
In case you're wondering, nigirizushi is not something most Japanese people would seriously attempt at home. Makizushi, maybe, but probably not in a mode you're much familiar with --- it's more akin to the giant futomaki rolls, and more cut in slices for kids' lunchboxes. Handrolls are where it's at for a home sushi party. Or chirashi-zushi, where you just pack a box with sushi rice and scatter a selection of things on top of that.