I had a 12" sub in my dorm room and I only got a couple complaints. It really depends on how receptive your floor and RA are to loud music. Actually, the people who were affected most were those living directly below me. My dorm had concrete floors, so there was significant structure-born resonance that rattled the cheap ceiling-mounted lights. So, if you have anal neighbors, it might be more trouble than its worth.
From a sound quality perspective, dorm rooms are about as bad as you can get. Limited placement options and highly reflective surfaces. You should take some time and figure out placement options before you purchase. Lofted beds give you a lot more flexibility. If you level-match your sub and keep the volume sane (especially at night and during finals) you shouldn't have too many problems. Keep in mind that not everyone agrees what volume is reasonable, especially if you don't know them well.