You should check your local regulations on your local sewer systems.
As far as I remember (someone correct me if I'm wrong) fixer can contain anywhere from 2000 to 5000 mg/L of silver compounds (silver thiosulfate complexes, Ag(S2O3)2 3-) when exhausted, it all really depends on what and how much you're fixing, whether you're repleneshing it, etc.
Different forms of silver have their own toxicity, and higher concentrations make things more toxic as well. The big danger to the environment is free ionic silver, which can kill fish, aquatic plants and the like. Silver thiosulfate is pretty stable, but if I remember correctly it can degrade to silver sulfide, which probably releases some silver over time (but not too much as it's pretty insoluble), but which is much less toxic than something like silver nitrate.
In the USA, the EPA has a sewer discharge limit of 5 mg/L of total silver (which includes the harmful ionic form and the less harmful thiosulfate and sulfide forms). Anything above than can be classified as hazardous waste. Obviously, this level is much lower than what your fixer probably has (I'm not sure how high it would get with just a couple of films/papers). Labs and the like are required to treat their fixer waste. Amatuers, as far as I know, aren't required to. Again, find out your local regulations.
Your local wastewater (sewer) treatment plant might have an activated sludge system, where microorganisms break down dissolved organics into stable compounds, water and carbon dioxide, as well as produce more microorganisms. Free ionic silver can kill these guys and upset the process, making the plant less efficient.