It's really not a problem to not shower for 3 days unless you've been riding crazy hard. At worst there's always the bum bath. And even if the BLM will allow gray H20 dumping it's still pretty gross. Eddie's truck is your best bet for a dump site
Dunefreak, it's not "not harmful" but it is no worse than anything else we all do out there as far as introducing new or harmful things to the environment. There is more in grey water than H20 and soap, such as bacteria from food. But I really don't think some bacteria will thrive out there and change the state of the dunes