This is definitely one of the areas I struggle with. I try to have the seams lock, and I try to be careful when laying out and pressing to see that they do. I always seem to come with up spots that don't lock and there's no way to press them to make them lock without taking something out...which I hate doing. When that happens I just stitch like they are and hope for the best. It usually turns out okay. I'm finding that when I piece/press in chunks instead of in rows it's less of a problem. May also be the reason I am becoming fond of sashing between blocks :) and seams that are off-set!