I don't trim the selvages until I'm cutting the pieces for my blocks.
Iceblossom, I've had red that ran and ran and ran, again, again and again. I discovered a product made and sold by Ritz Dye -- it stopped my bleeding problem, so I use it all the time. I also test all my fabric before I wash it -- if it bleeds, it gets treated. As you so correctly pointed out, it only takes one time -- I can't afford to have a quilt ruined by bleeding, and I can't afford to throw fabric away if it bleeds. So, I treat it, then test it again -- I've never had to treat fabric the second time, but discretion is the better part of valor, so I DO use color catchers.