I haven't used the washable markers directly on the fabric when marking a quilting design, but use them all the time on either golden threads paper or sulky solvy to draw my designs. For any marker, it should be tested on every fabric in your quilt...it may come out of some and not all. Best to be on the safe side.
I have used them to trace around plastic templated for applique and they did seem to all wash out when done.