It's an interesting consideration, isn't? How far does one go?
I used to think the bat-modulator should disguise Batman's voice to the point it sounded like a ransom-taker; and that's exactly what they did for BVS. I'm not sure how much it added or detracted.
For Hulk, or Professor Hulk, I considered the ultra low "ransom-taker," which is what Ang Lee used in the dream sequence (for me, in the only HULK movie that counts). Maybe that would be too funny after a while. Then I thought about Mark Ruffalo's own voice. Yes, it would be grossly at odds with his appearance but would we soon accept it because, after all, it IS the actor's own voice.
Right now I'm thinking a Clancy Brown or Ron Perlman type voice.