Well, if everyone uses a perm ink, it will be ok to sign before its washed. But, if you dont send the pen along with it.. you have no idea what they used.. and some inks will smear or wash out, whether you prewash or not.
What I do, when sending out squares for signature.. is this
Iron your square to a piece of freezer paper and draw a line on the freezer paper where you want them to sign..
Send the fabric on freezer paper, a pigma pen, and an self addressed, self stamped envelope for them to return everything to you, with a note to please return the pen along with the fabric..
This way you know what they are signing with and you can be sure it wont wash out.