Test on a fabric scrap first! I love Frixions but they can leave a faint white mark on dark fabrics sometimes. I tend to use them mostly for Redwork designs where I know I will be stitching over the lines( so it won't matter if they come back in the cold) and for small sewing projects.
Lots of people use them for marking their quilts but I haven't yet. I am leaning towards ordinary chalkboard chalk as I just posted on QB.