I haven't seen any rule about them and I think everyone is entitled to do what they like with their own work. Just a few pointers tho. a) if your map is based off of another with creative commons then its possible that its not allowed from the licensing point of view so maps for the CWBP should be free to use. and b) If you watermark it then I would watermark all the maps in the set and do it using a slightly different watermark as a straight up feint light text across it like some of them are easy to remove - the more so with the more amount of images using the same mark.