I've done it both ways that were mentioned above. There's small problems with each IMO. Precooking the potatoes to al dente and reheating to order and then adding dressing or whatever is fine, but you lose some of the flavor of mixing the dressing on the potatoes when they're warm, you know as the salad sits and gets cold. The dressing and hot potatoes has a chance to soak into each other creating something really tasty. When you do it this way it's really like dressing a salad. Good flavor but the dressing really hasn't soaked into the potato yet, not a bad thing, your dressing has to be well seasoned with tasty other bits mixed in.
The other way is good too, but when reheating an already cold potato salad. I find the potato to get starchy and very soft, depending on how hot you reheat and how long.
For me it would depend on what type of restaurant I'm working at. A nicer place, I'm going to reheat to order then dress. I would really play with different potatoes and season the water aggresively. Maybe add a splash of white wine vinegar when they are still hot after you drain to season a little bit so they're not so bland. And then add a nice fatty dressing to counter the vinegar. Fingerlings work really well for warm potato salads.