The MonsterFishKeepers site has a thread or two with much heated debate about this very same topic.
I have, honestly, never had a problem with canister filters, even AFTER extended power-outages (8-10 hours). I probably clean them out every 3-8 months... it just depends on my memory/time/etc.
The filter on my parents' pond is, effectively, a canister filter filled with bio balls. I know they have not cleaned it ever - water is crystal clear, fish are healthy, the only reason anything in that pond has ever died is because my dad is too cheap to plug the dang heater in during the winter. I think he's replaced the pump and I know he frequently cleans the pre-filter, but the pond has been running non-stop for about 10 years and hardly ever gets water changes. It is under-stocked and has plants, but not loads.
I'm no expert and I'm also pretty low tech. I hadn't bothered testing my water until recently and that's only to figure out what conditions my fish are breeding under (turns out that's highly variable, of course). I don't use many additives and, honestly, will top my tanks off without dechlorinating from time to time if it's just a little top-off. I've never seen any ill-effects.
I have trouble believing that canisters are the "nitrate bombs" so many people think they are - especially with a good pre-filter in use. Sticking a nice big pre-filter on a canister is like sticking an extra sponge filter in the tank.