I know several guys who have stuffed at one point or another. I did in highschool. It was no big deal. People always say it's stupid because when you get someone home they'll find out, but there are lots of times when its more about locker room pride, or just looking good hanging out with friends at the mall or the club. I say there's nothing wrong with a little padding, whether that be a rolled up sock or padded undies. If it makes the jeans fill out nicely, go for it!
Where I live, most guys wear REALLY tight "drainpipe jeans". I know lots of guys stuff because they usually have a nondescript round bulge and not THAT many guys have huge balls that magically stay in place all day long. Whatever, I think it's no big deal these days especially considering that even Calvin Klein is selling "enhancement underwear". The only thing is no self respecting dude will admit it.