I'm another big believer in prewashing and heat drying -- no softener or dryer sheets. I loved the original Dye Magnet but wasn't so thrilled with them when they went to the more papery substance, stopped using them. Since I mostly work with scrap and charm quilts, it's important that the fabric be ready for me to use when I'm ready to use the fabric.
When I'm washing unknown scraps pieces from a fat quarter up I wash with my general laundry. Smaller pieces I'll put several in a lingerie bag, but I don't bother with tiny pieces (which for me is anything smaller than 6" square).
I've only had one really bad experience with color bleeding, it was a reddish tone on tone, reputable maker. Also used a navy blue from the same line in the same quilt. I had prewashed once and didn't think about it -- until I washed the completed quilt top and the stylish maroon, blue & silver top with bits of bright white turned into bits of baby pink. And to make matters worse, some poly blend white squares had snuck into the whites so there were still bits of bright white. But that's why I wash my quilts before I give them away so if anything bad happens it happens to me and also why I purged all the poly blends from my collection!
I found another piece of that fabric at the goodwill, I've washed the second piece about 7-8 times now, it still casts off dye. I should dig under my desk where the two pieces are and take a picture of them. You would never believe they started out the same color.
All you have to have is one project ruined by bad dye or bad shrinkage to never want to risk that again...