The chemicals in the Frixion pens do stay in the quilt - not sure how much washing it takes.
Now many of us are not quilting for show - but here is a quilt show horror story that I heard last night at Harriet Hargrave's trunk show. Quilter completes quilt, removes lines with heat. Quilter later enters quilt show in Minnesota. Ships quilt which travels in belly of plane - sub-freezing temperatures then is in postal warehouse which is also quite cold on the way to the eventual show delivery. All marked lines reappeared and were there while quilt hung in show.
I also don't know if the chemicals will eventually harm the fabric. I'm still figuring out which I like best but prefer things that I can brush of the surface like chalks or pastels.
Cheers, K