Reminds me of the old lady that said she didn't care how high the price of gas got, she never put more than $20 in at a time anyway!!!!

 

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 only fill till the pump clicks.

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.