If it were my fabric wing, I'd be painting on the registration letters with the same type of paint or dope used to finish the wing. Though I can't show hard evidence, I have certainly seen stickers on fabric, where time and exposure has caused the sticker to shrink and crack the underlying paint around the edges.
If you must use a sticker, I'd be contacting the manufacturer of the paint or dope, and get their recommendations...
Pilot DAR