I do not prewash pre cuts and mix them with prewashed fabric without issue. In fact most of my quilts contain both all the time. My criteria for prewashing is -- does it smell funky? Does it feel funky? Is it a bleeder? Is it loosely woven- hoping to tighten it up a bit. When I use hand dyes, or deeply saturated colors of batiks I prewash them, they are the ones most likely to bleed. I launder my finished projects.