This is one of my concerns, too, along with the fact that I live where it is frequently below the 14ºF 'minimum temperature' at which the markings reappear...as do the people I give quilts to (yet, to the best of my knowledge, there are no Eskimos in New Hampshire).
Frixion pens have never been recommended for fabric use by the manufacturer and if they don't have enough confidence in their product to do that, I sure don't. Sticking with my trusty Sewline ceramic pencil which WAS made for fabric use. To each their own, like so many other decisions in quilting.
Well, as usual, I can count on you ghostrider to give us great information. I've been scratching my head on this one for a while...to use the Frixion pens or not. I've been using a Sewline and I think with all things considered I will stick with that. There are so many other things I can think of to spend money on. Thank you, again!