It depends what you want.
Bitter orange peel added to the boil does not really add orange flavor so much as a pithy acrid bitterness to the finished beer. This is the way we made witbiers 20 years ago. I'd use restraint, as more peel won't add more orange flavor but just more pithiness. I'm not sure what it would do at end of primary. A vodka tincture might not be a bad idea, though I don't know if I've tried it. Is the use of dried bitter orange peel "right" or "wrong"? I actually don't have an opinion on that. However what I will tell you is that I might no longer use bitter orange peel in my own witbiers anymore, in favor of.....
Fresh zest added to the boil might do something, but not as much as if added to the end of primary or secondary. However I would still recommend restraint. It's been a while since I've made witbier but I'd guess the zest of one orange might be too much for 5 gallons. Maybe I'd use just a half of one. I would not add the whole peel, but just the zest. I would recommend a tincture in a few ounces of vodka, let that soak for a couple days, then add just the flavored vodka to taste at bottling/kegging time.
The predominant orangey flavor in a witbier comes from coriander. I'd add that at the end of the boil, but a vodka tincture wouldn't be out of the question either. Again, use restraint. The stuff is powerful and easy to overdo. If not sure how much to use, take a guess but then only use 1/3 as much. It's much stronger than your brain tells you it is.