I've tried different methods, and for me, once all the fabric is sewn to a piece of paper, I trim them to the seam allowance. I don't trim the outer edges, just the sides that will be sewn together to form the block (like 8 pieces to form a star block) . Then I remove all the paper. I've found I can't always get the lower piece to feed exactly how I want. This gives me a bit of flexibility to match up seams and points where I need them to be.
Once I have all the pieces of the block together, then I'll trim the outer edges to get the correct unfinished block size, being careful about points so they don't get cut off.