I learned the hard way...I washed (well, in the sink) some fabric for a Dear Jane swap last year -yes, red- then I proceeded to make my set of blocks...18 were due. I had 11 made...had spent hours and hours on them. I laid the pile on my ironing board (which was damp from ironing something else) and I went to bed. The next day my ironing board was red!!!! and the white on the blocks had splotchy pink spots...I rushed them to the sink...they ran..and ran...and ran...wouldn't quilt.
I threw them in a drawer..threw away the rest of the red fabric...WASHED more fabric in the machine and started over.
YES...I always wash my fabric now...and I give Color Catchers with each quilt I give away.