Dumping it down the darin can cause corrosion problems with metal piping--if you don't flush several gallons of freshwater afterwards to clear the SW from the system. In the winter, you can save it and use it to remove ice from sidewalks and driveways.
I use the same bucket for changes from my FW and my nano SW, and then use that water for house plants and the gardnes--but there's about 30 gallons of FW and only 2-3 of SW, so the salt is pretty dilute by the time I use it. If you dump it on your garden/yard, water regularly, and spread it out, I don't think it would cause a problem. Of course, you'll want to check the pH preferences of the plants, and make sure you're not dealing with a plant that requires miccorhizoids (sp?) to prosper. The SW will alter the pH of the soil, in the long run, and could be lethal to some molds and fungi.