I asked our post office today and they looked through all their terms and conditions.

They spent a lot of time on it and found all the regulations as to where stamps are to be placed, where address should be etc.

But no where does it stipulate stamps cannot be taped on. In fact my post office and others encourage taping of stamps as well as taping stamps themselves if they may fall off or don't adhere properly and firmly believe that an item will not be returned on that basis.