I never prewash and always use scant quarter inch seams. Depending on the quality of the fabric, I sometimes use Best Press starch alternative. This or starch should be used before cutting. If my blocks need a little squaring up, I use Best Press on the back of the blocks. Be sure blocks are completely dry (if you use starch or steam) before squaring them up. My Juki 20-10 came with a 1/5" foot but it seems that's the only machine that has one. Its an exact scant quarter inch. I have tried to find them to fit my other Juki and my Singer but they do not seem to make them. The Juki 20-10 is considered an industrial machine so I guess that's why. But even with my other machines I always try for a scant quarter inch, which allows for the fabric taken up by the seams. I'll have to try Rob's tip of measuring with an index card.