Depends on the basil freshness/quality and the situation I'm serving in...
If at a private event or just for my own family/friends, I'll do post bake for a fresher taste of basil.
If at the market, I always pre-bake under cheese for two reasons:
1. Basil pre-bake helps with masking the basil if it's not as fresh as you'd like. Even a little bit of limpness from basil will show significantly when you're doing post bakes. I don't want customers feeling like the basil quality isn't the best, so pre-bake helps meld the flavor and allow the pizza to shine better as whole.
2. My local health department insists that if we do basil post bake, we have to wear gloves. With limited staff (three people - one cashier, one pizzaiolo, one fornino), gloving/degloving for every pizza with basil is just unfathomable.