I do believe in prewashing - everything - but there are plenty of quilters who don't. I don't worry about the strings that can develop during washing. If I think there will be a lot (as with FQs rather than yardage) I may just soak and spin the fabric and skip the agitation cycle, where much of the fraying occurs. However normally I just cut or rip off the strings. Not much fabric is lost that way, and it doesn't offend my sensibilities to have the occasional tangled mess. I prefer working with fabric that will not bleed, that has already shrunk all it's going to, and that doesn't have the chemicals and sizing in it. (I'll add starch if necessary.)