I voted "It Depends" ... If I already see that I'm going to be cropping a section I do it right away. Occassionally I come across a situation where I've finished post-processing and have to crop for a specific size requirement (800x600 for example) or feel that a little nip-tuck would produce a better photograph
I try to get what I want to work with straight out of the camera - but when I do crop, I tend to do it before processing. However, sometimes you get the situation that AfterImage mentions - someone needs an image that is a certain dimension, and you have to crop your 'final' output to match their dims etc
I'm in the "it depends" category, too. If I'm just processing in Lightroom, then I usually crop/straighten first. If I'm processing in Photoshop, especially if I'm using Noise Ninja, I usually crop near the end.