I don't have much to add to this discussion except to agree with goo and to 2nd what Dale said. Please DO check to see what's in the queue already BEFORE you request unless, or even if, you plan to be listening 4+ hours after your request and really just HAVE to hear THAT tune by a band that's already in the queue regardless of how long the queue is. It will save you some angst at not being able to hear your req for so long
I personally req very seldom because I find that I usually hear more good new music by listening to others' requests. However, if the queue is SHORT (i.e. a few tunes or less), please do request because as intelligent as SAM sometimes seems to be, humans generally are better at picking music than she is. As for planning your day around your request, I imagine most of us have better things to do and few of us have the luxury to do that even if we wanted to. Anyway that's my $0.02.