This question pops up a lot, and like many things in the world of knitting, there is no stiff, permanent rule. However, In most cases, I’d recommend blocking the pieces of a garment before sewing seams. Here are three reasons why:
- If two pieces that need to be joined end up slightly different lengths (in a perfect world this would never happen, but I can attest to that sneaky row gauge mysteriously changing halfway through), you can work a little stretching magic when blocking your knitting to help the pieces match up when it’s time to sew.
- If there’s stockinette fabric involved, it will help tone down the amount of curling at the edges, making sewing easier afterwards.
- Sometimes blocking for knitting can significantly change the fabric (some textures might lay flatter, and lace patterns will open up) so sewing before blocking might lead to unforeseen problems like puckering once the item is blocked.
I’d like to point out that there are plenty of exceptions to every rule, and in certain cases it might make more sense to wait to block until after the seams are sewn. Use your best judgment and get sewing!