This is what I’ve picked up from reading threads on the topic.
The reasons some people prewash are:
- To remove the sizing from the fabric. The fabric feels and hangs differently after the sizing has been removed (which can be considered a good thing or a bad thing, depending on your view). I’ve read of at least one person being sensitive to the chemicals.
- To remove any dirt and oils that have gotten on the fabric
- To shrink the fabric. Different fabrics (and especially different fabric types) shrink different amounts, so it can help reduce uneven shrinkage in the finished quilt.
- To see if the fabric bleeds. Red is the only color that I’ve had problems with myself (and of course not all reds). Some people wash all their quilts with color catchers, and so are not concerned with bleeding.
With pre-cut pieces of fabric, the shrinkage can cause problems if it leaves the pieces too small. I bought a kit for something once and it left the pieces too small, and I had to make some adjustments. Raveling is also more of an issue with smaller cuts since that can also leave the pieces too small. If you do decide to prewash small pieces, putting them in a lingerie bag can help with the raveling, or you can just swish them around in water or soap and water.
I do prewash fabrics in my washing machine, but for anything smaller than maybe half a yard I would either skip it or hand wash them.
In the end it's just a preference, but I hope this helps in deciding what you want to do.