Doing it before they begin leaking saves water and money. Test for toilet leaks by adding food coloring to the water tank. Don't flush for 15 minutes. If there is color in the bowl after 15 minutes, you may have a leak. Don't use the toilet to dispose of tissues or other trash.
Every flush uses three and one half to seven gallons of water.
When replacing fixtures, install waer saving models. A bathtub filled half full holds about 50 gallons of water. Consider showering instead. Limit the length of showers to two or three minutes. An unrestricted showerhead runs at five to ten gallons a minute.