I marked some pieces for applique (a number coordinating with sections of the pattern) less than two weeks ago. I ironed them yesterday. Sure, the dark ink came off, but left a white number on the fabrics! I don't know if it will come out in the wash, I haven't sewn the quilt yet. I had already pre-washed the fabric.
Luckily, I marked the pieces on the back, and it doesn't appear to have gone through the entire fabric.