For my own use - whatever works. For a group project - probably 1/8 to 1/4" depending on the size of the block. But maybe that's why I seldom do group projects. When I first started quilting I somehow wound up putting together all the blocks from a group, and it was for a quilt meant for a raffle. I wound up having to make extra blocks to replace the ones that just would not fit. I wouldn't want some other quilter to be saddled with blocks that I didn't have the skill or patience to make well.