I feel like it really depends on your perception of the draft class. I always think of classes in terms of pyramid drawn by a Second Grader; it's skinny at the top, wide at the bottom, and none of the lines are straight.

 

What I mean by that is there are always "top" guys in a given class who are just better than everyone else, but there's just a few of them. As the draft goes on, you get into the "fat" part of the class where players all tend to have similar value/potential. If you feel like those top guys are already gone, and you feel like the draft is getting into the fat area, then you trade down. If you feel like there are still difference makers available, then you don't.

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