It's not in the regs. But after dealing with game wardens on a personal note they don't like having to wait around for someone to pack out the rest of an animal just to verify the tag and sometimes they get very testy about it. So it's best to bring the quarter with the verified sex in first and attach the tag on it. It's not a rule it's more of a common sense thing with dealing with game wardens. They may eventually come back to your camp to see if it was a legal bull. Most game wardens are not going to accompany you up the mountain to see a kill. They will either wait for you at the pickup or camp or just move on. If you are worried about someone stealing a quarter from you while it's in the truck or hanging in camp. I do have that answer. The game warden will tell you it should of been tagged regardless of worrying about someone trying to steal it. The tag is useless anyway if someone steals your game. They can always issue another tag if that happens. It is always better to have that tag first time out that's common sense you will see the game warden on the road or at your camp but if you see one in the woods in Colorado during the hunting season please take a picture for me, because they have a tremendous amount of roads to travel in one day to be off walking in the woods. They don't have time to play games during a regular hunting season. And you will likely get a ticket.