The problem isn't always bleeding when there's color transfer. It can also be crocking. Crocking is more likely to occur when you have a larger load (e.g., a full sized bed quilt) than just a piece or two of fabric, but rubbing with a white piece of fabric/cloth while it's both dry & wet will usually give you a decent idea. Hand dyed & batik fabrics are more likely both to bleed & to crock, so it is a good idea to pre-wash your fabrics if you will be working with either of those.
The other thing to consider is if you are using different types of fabrics together, they might shrink at different rates & it can be helpful, therefore, to pre-wash everything to reduce how much each fabric will shrink once the quilt is complete. However, most modern-day commercial quilting cottons are made & dyed/printed in such a way that they shrink very, very little & are unlikely to bleed or crock when washed properly (cold water, slow speed with dye-free detergent in top-loading washer; tumble dried on low heat). It is, of course, the absolute safest option to pre-wash, but I put it in the same category as giving your purse a good disinfecting inside & out every evening. Sure, there's a good argument to do so, but who has the time?!!