I've heard others do that as well aldo but with that a4 clear sticky paper stuff. Personally I just stick my tax disc in my wallet. Spoke to a copper once looking over my bike and said if u need to see tax disc it's in my wallet. His answer was he wasn't bothered about the tax and was just having a look.
Bay, that might be OK if you catch the copper who is looking at your bike, but if he decides to report it & you are not there, it might be a load of chew.