I've crossplayed quite a few characters. I'm not that big chested so I usually wear a sports bra. The characters I've done typically don't wear tight clothes, but more "fitted." But yet, it's a little loose around the chest so that it appears flat. Like, I'd say wearing a Kimono/Yukata doesn't require heavy binding. The other time I actually somewhat binded was for an open-chest cosplay. I wore a sports bra and binded it with ace bandages (not too tight but good enough that it was flat) before wearing a skin colored shirt on top. And it turned out fine.
But of course! As others have it, it depends on your breast size. Best to go with a sports bra and then work your way around it! It would also depend on the character.