Regulations vary. Check MSDS to see what's in your chemicals and decide what to do.
Most common ingredients that are in most regulatory lists are: hydroquinone, borax, boric acid, metaboric acid, metaborate, etc. Even if not regulated, it is generally a good idea to avoid dumping chemicals that contain these agents.
Silver in exhausted fixer makes sludge once it enters the sewer. Apparently some think this is a burden to the treatment plant. It is also a waste of silver, which is limited resource.
Selenium toners are better to bring to hazardous waste collection.
Dichromate, ferricyanide, cadmium compounds etc. are more problematic. It's best to minimize the quantity used, and also store the exhausted stock for proper disposal. (Label each solution with the list of ingredients from MSDS! It's not always good to assume that the disposal plant will figure out what's in it. Also, this info will be very useful in case of accidents during storage/transportation/etc.)
Cadmium and mercury compounds are more serious. Be very careful with them. Mercury is very reactive with many kinds of organic matters and also bacteria to make more toxic compounds than inorganic mercury salts. Proper management of mercury compounds is rather cumbersome even in real scientific labs. It is simply best to stay away from these agents.
If dilution is the solution I've got the
problem licked. I use all chemistry, film
or print, very dilute and one-shot. Dan