Concept, Character, History: Traditionally, it's beef sashimi, not tataki. However, the tradition on the dish is not all that long; it dates from mid-20th C Venice. There are pluses and minuses to tataki style for slicing and pounding -- but if that's your choice, why not? Concept: From a sushi point of view, think o-toro richness with hirame sashimi presentation.
Garniture: Dressed with olive oil, sometimes infused, shaved dry cheese such as parm, capers, garlic chives, bitter greens, chili flakes, yadda yadda yadda. Highly adaptable to sashimi presentations and garniture. Again, think toro taste and hirame translucency. Anything that goes with negi-toro goes with carpaccio. Got scallions? Lan-yu would be interesting -- but tread lightly young padwan. Grated, spicy radish could be interesting. If you like wild fusion, I think an XO mignonette would be killer. Dam, talked myself into it. I'll put that together this week.
Meat: Fillet. You don't want too much marbling. Wa-gyu is not the best choice, too fatty. Ideally, you're looking for something like "Better than Choice Angus." Grass fed beef has more flavor. You can use any other cut without grain you like. Top block sirloin is an excellent choice. You can even use eye of round for an economy carpaccio. Okay, that's pushing it. Don't.
Knife technique: Big deal. Use very cold meat, but not frozen. If you've got the time, tools and technique you can do a better job by hand than with a slicer. The slicer's blade is toothy and tears the meats fibers instead of cutting cleanly. You want a polished edge -- right between "near mirror" and "mirror." In sharpening terms, a Hall's black Arkansas, Norton translucent Arkansas, an 8000# Norton water stone, a 5000# or 8000# Shapton, etc. In other words... sharp. You want enough flexibility in the blade so you can feel the face of the meat before cutting through it. A suji or yani that's not too stiff are OK. I guess a tako or fugubiki would be ideal -- but have never used either, so as I said, a guess. With my knife (old French carbon slicer), I cut straight down to the board, no bias whatsoever. I know that's not sushi style. If your thinnest technique is on the bias, do that. Fillet doesn't have much grain structure anyway. Time consuming, neh? I have fairly fast knife technique and it would still take me awhile to cut each serving.
Pound: Yes, slightly, no matter how thin you've sliced. The idea is not only to thin but to enlarge the slice, give it an irregular shape, and to open the fiber structure so it is translucent and takes the dressing. There's actually a right kind of pounder for this. You want to use a flat, disk shaped pounder on a vertical or offset handle. You pound straight down, then drag the disk across the meat to stretch it. The whole process is facilitated by pounding between lubed sheets of cling film. After pounding, hold the slice up to a window or light to check. Really. I'm serious. Makes a good show.
Freezing: Better if you don't. Alters the cell structure, screws up the final texture and moisture content.
My two lira,
PS ON EDIT: FWIW, tataki means quick seared on the outside, raw all the way through, and served chilled (or at least cool). I used "sashimi" to mean raw.