pre-cuts are always an issue when it comes to pre-washing/not pre-washing. ...
most of the shops, manufacturer sites will tell you with today's fabric making techniques it is no longer necessary to pre-wash. pre-cuts are cut with the belief they will NOT be pre-washed. they will shrink a little and no longer be the pre-cut pieces you thought you had. some people no matter what have to pre-wash, if that is the case i would avoid pre cuts at all cost and if somehow it was necessary for me to use a pre-cut i would put them into a mesh bag wash very gently (just enough to remove the chemicals/sizing in the fabric then i would wring out well and iron dry.
when i purchase new yardage i pre-wash it if it is a color that commonly bleeds (reds, blacks) i pre-wash if the fabric has a funky smell or a funky feel to it. there is something very nice about the feel of freshly laundered cotton fabric. then press with a little sizing or starch if needed; but if the fabric is damp when you are ironing it you should not have to add anything.
oh and i ALWAYS pre-wash if it 's batik's or hand-dyes.
i don't worry too much about it if it is a printed fabric from a (big) manufacturere-but i do test to make sure nothing is going to run.