Do what you want, unless you are at an airport that expects you to dump them in their buckets. Then follow the rules, but I have never been to one of those airports.
Many FBOs over fill tanks and as the heat of the day expands the fuel, it tends to over flow and run on the ramp. Go to an old cessner parking spot and draw a line from the fuel vent straight down and you will find cracked asphalt. That is the bigger problem.
The way I figure it is that I can either throw in on the asphalt and let it evaporate in the atmosphere or put it back in my engine and release it into the atmosphere via combustion emissions. either way....