If the pre-cut is already cut to the size that you need (e.g. it's a charm pack and you need 5" squares) then you just use it as-is. If your pre-cut needs to be cut down anyway, and you won't be losing use of much of the fabric by cutting off the pinked edges, then I would cut them off. It's harder to get a good consistent seam allowance when you're dealing with those ragged edges.
(What I mean by not losing much of the fabric is - suppose your pre-cut is 5" squares, and you need 2 strips that are 2" x 4.5". You can cut off the pinked edges and still have your two strips. But if you need 2 strips that are 2.5" x 5", then you really need to just cut the pre-cut in half, because if you cut off the pinked edges you won't have enough left for your two strips.)
Hope that answers the question.