I dump fresh water anywhere where it won't make a mess (mud etc). Usually don't have to because I use it enough-maybe once or twice a year at most to dump fresh water. My favorite is to dump it on my paved storage lot when it's raining. It minimizes confusion that I may be dumping something other than fresh water. Letting it run down my street in front of my house may generate undesirable and unnecessary concern.
Another tactic if the above doesn't work for you, just run faucets till all the fresh is in the gray tank, and dump that normally.