Keep in mind, the waste water has already been through the sediment filter and the carbon block filter, and in most cases, even if it hadn't been through those, it's only got around 1/3 more TDS than the incoming tap water...
I.E. The "waste" water isn't THAT bad, and there's a decent chance it's STILL better than tap water (I haven't tested it's actual TDS yet, but will be doing so shortly)
I considered running it outside to the pool, or to the foundation of the house, but my concern was that, while it doesn't happen often, we DO get freezes here in Texas, and having my waste line freeze up wouldn't be good for my RO/DI system at ALL!
My current plan that I'm tinkering is to have the waste run to a water dish for the cats, then have a level drain in that water dish that drains the overflow to a drain... Most of it will still be wasted, but at least the cats will always have fresh moving water to drink from...
I'm toying with some ideas on draining the overflow outside to water the foundation, but I'd have to have a way for it to overflow and still go to a drain if the line going outside clogged up or froze up...