i only prewash if there is dye showing when I rub a white paper (very hard) on a fabric. I was told to do this by an LQS owner. i don't have a sensitivity to dyes,etc so no concern there. I don't worry much about shrinkage cause I use lots of steam when I iron the yardage so it shrinks then. I think a lot of the shrinkage that gives that "vintage" looks is because there's shrinkage with bat and backing (although I do wash backing often cause I find wideback needs to be in order to get it straight). I like to hand my washed fabric on the line to dry--less wrinkles!