i think this all depends on your personal style and how you like to mix.
i know a few tracks that start off great and end poorly so i like to mix those out early, on the flip side there are plenty tracks that i have to let the whole track play too.
on a sidenote – i hate it when a dj cuts a track i love short, depends on the nite and incoming but it its a good way to get me to take a break from the dancefloor occasionally ^_^