Never been an advocate of pouring it back in the tank - might there be some sediment in the receptacle which is being used which might end up in the fuel system?
I have asked our local GA field to provide suitable receptacles near the hangar for the purpose of disposing of fuel samples but have yet to see any! I believe that pouring samples away on the ground may be technically illegal and it doesnt seem very eco-friendly. Also it can erode tarmac if thrown away on such surfaces.
I was told that in USA you can be subject to a fine if you dispose of samples on the ground.