normally you put them on the intake hoses and return hoses so you can stop the flow of water. also you might want to install a check valve on the pumps return line, they are sold at lowes etc..
Your sump should be setup so that when the power is off, it won't overflow. Small holes in the return lines will form an airlock as they siphon water from the main tank. Cleaning is then as easy as unplugging the return pump. Don't rely on a check valve. It will fail eventually, and you won't be home to deal with it.
Check valves are fine and reliable. I got 2 very nice ones at home depot used for sump pumps for houses for 10 bucks each, and there is literally no way for it to fail unless you physically abuse the things. Theyre very simple to install as well.
One piece of plant or a scrap of driftwood and the valve will not close enough to seal. If that is your only way to ensure your sump doesn't overflow, you're screwed. It best to setup the sump properly from the start and avoid check valves all together.