I don't prewash because I always want to jump right in and get to work and I hate having to press all the yardage I buy.
I do however cut a small corner piece 1 or 2 inch square. Put it in a cup of water and pop it in the microwave for 60 secs. Let it set a few minutes, go back and if the color has any color at all I wash the piece. When I press my fabric after washing I always use spray starch or sizing to make cutting and measuring more accurate.
I have washed maybe 3 pieces of black and 2 pieces of red and I have made over 130 quilt tops.
Good luck whichever way you go.