I've actually played TF2 between 90 and 120 FOV. 120 is excessive and makes it hard to aim. I've found that 100-110 is optimal - whenever I have to go back to 90 it makes me vomit.
I think 100 would be a good compromise. 90 is just a little too low, but any higher and you'll compromise gameplay, like scouts and spies.
Source: I know an exploit that gives me anywhere from 90 to 120, user-selected, that can be performed even in sv_pure 2.