I prewash 90-99 per cent of my fabrics before using them in a quilt. I like to remove the fabric from the dryer while it is still somewhat damp, which makes ironing easier. That said, in a pinch, I have washed 1-2 yd. pieces in the sink, mostly to make sure they do not bleed, particularly if I don't have a lot of fabric to use the washing machine. While this makes for some extra work, I have been surprised some times by some dark fabric which doesn't bleed, as well as somewhat lighter fabric needing to be rinsed out 4-5 times. I think the debate of wash or not to wash will be going on forever, just do what you are comfortable with. You can mix washed and unwashed fabrics in a quilt, particularly if you are using pre-cut fabrics, which I do not typically prewash.