Yeah, you have to always remember that the markers can be heat set even if they're supposedly washable. So don't iron over the markings!
My favorite is the air erase markers. You don't even have to wash. Just wait a couple of days and the disappear. Of course, this means you have to mark and quilt on the same day! But they are awesome, have never let me down.
Test any pen on a hidden zone of your quilt.