It's rather pointless and costly to get your nitrous oxide from whipped cream canisters at the grocery store. I'd need a good three of them, just for a few minutes of high, at comparatively high prices.
I'd imagine it's not likely to be damaging, though. Just wasteful. It's not going to be pure nitrous, either, it'll likely have other gases in it - which, I guess, could be dangerous depending on which ones, though not very severely.
Also, to the person who said 90% remains after nitrous oxide inhalation, you're correct. You can inhale the stuff and then exhale it into a balloon for re-inhalation, keeping the high going, from one "use". I read that it was 95-99% expelled in your breath, with possibly as little as one percent reaching your circulatory system, and much less reaching the brain. Theoretically, you can use one dosing of nitrous oxide, and by conserving the gas when breathing out (ballon or otherwise) and re-inhaling, stretch the high for quite a long time.