I think the "harm" is risk of overflow. If there is no room for expansion with a hot engine warming the gas in the tank it will need to expand somewhere. It will take the path of least resistance. I always try to stop at the bottom the tank mouth insert.
I used to have a Kawasaki Vulcan 1500, the gas cap had a valve built into it to vent expanding air inside the tank. On a warm day or after a good ride it would literally whistle.