To me, the concept or "spirit" of cheating only applies when you're competing with others, whether that is in a multiplayer game, or comparing notes in a singleplayer game. So in that sense, it is technically possible to "cheat" in a single-player game, if you're telling someone "Yeah, I reached the end boss, have you?" But then fail to tell them that you used a clipping hack to slip into his fortress at the start of the game, or what have you.
Anyway, I don't recall ever cheating in a multiplayer game, which probably* means I haven't, as my brain is adept at not letting go of guilt, and I would definitely feel guilt about it. However, in a lot of singleplayer games I'll sometimes use "cheats", especially if it's a game where I'm not playing it for the challenge of memorization or repetition or whatever less-than-fun obstacle I'm looking to bypass. Since I grew up in an age where you couldn't really "game" for a living (unless it was chess, poker or some form of billiards), I never looked at singleplayer games as an ethical dilemma. I had no qualms asking my friend Tim how to get past the Babel Fish puzzle in the Hitchhiker's Guide To The Galaxy text adventure. Nor do I feel any remorse over not figuring out that stupid, stupid cat/moustache puzzle in Gabriel Knight 3 on my own. So if that makes me a "dirty dang cheat," then so be it, as long as it's understood that I'd never do that to win against another human.
(* - I say "probably" because I've been playing multiplayer games since the first Atari Combat cartridge, so that's a lot of ground to cover. And I can't swear that I never glanced to the left of my screen quarter in Goldeneye.)
Is there a special sort of pride in making it through a difficult game with zero assistance or clues? Sure. Is it an absolutely necessary part of gaming? No. Not for everyone, at least. Maybe it is for you, and that's fine. But games are just that, games. It's not real life, so if you want to give Bella Goth 5,000 Simoleons that she didn't earn, go for it. If you want everyone to have giant bobbleheads, have fun. If you're not having fun, what's the point?
"If The Great Gamemaster In The Sky didn't intend for us to cheat, they wouldn't have given us Up Up Down Down Left Right Left Right B A."
A Super Mario Bros. fan who ALWAYS used the warp zones.