Back in the day, I used to like the top bunk. That changed my first semester at IU when I had the roommate from hell. Among other charming things she did, she had guys call her at all hours of the night. That wouldn't be so bad except that she slept right through it, and the phone woke me up. So I always ended up climbing down from the top bunk to pick up the phone. It was annoying!
When she moved out I moved down to the bottom bunk and have been a bottom person ever since.
I was never in a dorm with triple bunk beds but I did go on camping trips at a place where there were triple bunk beds. I took the middle bunk...the top bunk was just WAY too high for me.
Funny story: when I was at this camp on a fourth grade trip, one of the girls in my class flipped over her mattress and found that someone drew a large, anatomically correct picture of a naked man on the other side of the mattress! Needless to say, she freaked out. Of course all the rest of the girls had to get a look at it, but no one would go near that bed the rest of the time!