I do not currently have guy roommates, but I have in the past, and I've had guys as overnight guest (you know, the kind who aren't in my bed as per relevance for this question) so I'm going to answer.

I don't mind walking around without a bra around guys, even when I am careful to wear a certain amount of minimal clothing (more than most of my friends, as it so happens). But so long as my clothing isn't see-through, I don't mind wearing no bra. But that's probably because I'm just a B.