Although I have used Frixion pens and I really like them a lot, I am cautious when using them. I've never had a problem with the ink not washing out, but I'm careful to only mark areas for cutting, which would be hidden in a seam if it came back. I don't use them for marking quilting designs.
Yes, our homes are warm inside, but what if I made a quilt for my granddaughter, wrapped it for Christmas, and took it to her house to give it to her? It is often below 0 here in Wyoming, and just getting a package into the car exposes the contents to very low temperatures. Have you ever taken a quilt to a high school football game? That could also expose the quilt to temperatures low enough to reveal the markings. The point made about quilts being sent to shows via airline is another situation that we need to consider.
I won't stop using Frixion pens, but like I said, I am cautious and suggest that we all use care when marking our quilts.