I follow the simple rule of leave no trace behind-
If I am in a situation where I can let the grey go and leave no trace- I do it. And I do it regardless of what the regulations might be. If my grey helps a tree or shrub be a little more comfortable during the drought- then I am willing to take the risk.
As a side note, I also leave any extra fresh water behind as well. Generally I let the grey and fresh go the evening before we depart just so I can be sure everything has percolated into the ground. I don't leave a puddle, I don't leave a damp spot, I don't leave any sign I was ever there.
Just to be clear, I always practice the above in a manner that is respectful of the next camper and always leave my site in better shape than I found it!