i guess it depends on your set up. my eheim comes with "quick disconnect" joints that are basically two ball valves on either side of a threaded union. (
basically this, though you could find cheaper ones, or even make it yourself.)
I don't lose any water, except the quarter ounce that gets stuck between the ball valves. shut the ball valves, disconnect the unions, lift the filter and hold it over the aquarium, open the bottom ball valve (to release pressure), then the top valve (to break the vacuum) and drain the canister.
Then you can do all your maintenance with barely a drop spilled. I like to pour the filter media into a bucket of tank water to clean it.