Outside of using the dish water to put out the fire at night, I believe in take home what you bring with you.
Now if you are out in the boonies and toss a pan of dish water in the bushes, just make sure you don't hit the guy taking a wee next to you. There is a big difference between that and draining a tank. I've seen the results of not just grey tanks being dumped out in the wild, it's disgusting. Someone else will camp on the mess if they know it or not. If you wouldn't dump in your front yard, please don't dump it in what I will call my yard for the next week.
Otherwise check with the ranger station or the BLM office in the area if you must dump grey water.