A lot depends on the contrast of the fabrics you'll be using. If it is a high contrast I tend to prewash everything. That said, I have a red/white/black quilt in my pile that did not get prewashed. I will simply throw 4 color catchers in the wash when I do wash it.
You can do a color test on your fabrics to see if they will bleed. Either snip a piece of fabric off the edge and put it in soapy hot water or take a wet Q-tip and rub it over the fabric to see if color comes off.