I use frixion pens all the time. Yes the lines do come back when frozen. The reason I know is I sent hours marking a piece then ironed it on accident. Everything disappeared. Then I remembered that if frozen it comes back. I threw my quilt in the freezer for an hour and wala it came back.
But I also heard that a good washing take the marks away.
Since I live in So. Cal I don't thing freezing is a problem.
I will continue using them.