I have a septic system at my house. I've read here on APUG that there is probably enough sulfide to bind the silver into insoluble silver sulfide and I don't doubt that but have nevertheless decided to "play it safe". I'm not a chemist, and that ignorance factors into my decision not to gamble with the groundwater my family drinks or the functioning of my septic system. I am perhaps being slightly overcautious, but it's no trouble at all, so who cares?
I've asked around at the one local film photography store and the local photography club and there are no photo labs or anyone else in the area who still take used fixer for recycling, so that's not an option where I live.
However there is a convenient hazardous waste disposal program in my county. They specifically do take used photo chemicals. They take up to 5 gallons per visit, it's drive-thru and only takes a minute, and I drive by there several times a week anyway. It's simply no trouble and I've spent more time reading/writing about fixer disposal on APUG than it actually takes to do it ( a gallon dropped off every few months ). Not much more time than it's taking me to type this post.
Like NedL above, I take it up to our local hazardous waste disposal facility every couple of months. It takes very little time, costs nothing, and I believe it's better for the environment and local water treatment plants. I would not dump it down the drain.
I haven't done more than 2 gallons probably. But good to know. Since I've gotten back to traditional Photography, that part I forgot about. just trying to find a small portable storage container that will fit horizonal under my sink to pour into.