I only machine piece and machine quilt. The first quilt I ever made, I pressed the seams open and when SITD, I could see some of the piecing thread in the seam. In the first washing, that quilt also busted a seam. I've pressed to the side ever since then and never had another problem.
Have heard great things about the clapper, but not wanting to spend money on another gadget, I found a great tip on a blog (which I can't remember which blogger it was so I can't give credit, sorry!) that if you put a stack of magazines on a freshly pressed seam, it gives a super flat press once the fabric has cooled. I sort of assembly line mine, where I can usually press 3-4 seams and have piles of magazines on each, and then rotate to move them off one by one to my quilting table to continue being flattened until completely cool. It's working out pretty well!