I don't prewash precuts, but I do prewash my yardage cuts. I have started using Shout Color Catchers in my front loader to help corral any errant dyes. I run my washer on the shortest cycle and then into the dryer they go. I do have a batik quilt that I didn't prewash because I didn't have the Synthrapol (or was it Retayne?) for the prewash when I was ready to start. I have both now along with the Shout Color Catchers so I am in good shape from here on out.
I know some people don't prewash, but I prefer to do it so I don't have to deal with uneven shrinkage when the finished quilt does go through the wash cycle. Just my preference.