As w/ anything - always test it on scraps.
Now, w/ that out of the way - I LOVE LOVE LOVE the frixion pens. Thin lines and if i make a mistake, i can take my iron to it and it disappears and allows me to start again.
I always wash my quilts after i am finished, so i'm not worried about "anything" that may be left behind. Prior to learning about the frixion, I just used lead mechanical pencil. By the time my hand/arm got done rubbing against the quilt as i was working on it, there wasn't much left on the quilt.