I'm not pure, but I stay reasonable.
I play for Pokédex completion, and it doesn't feel right at all having NPC congratulate you when your Pokédex is objectively not complete. So, yeah, I got myself GBA and DS Action Replays, used them to get the event-exclusives, and put them away. All my other Pokémon were caught in a 100% legit manner... yup, even that (insert insult here) Feebas in Gen 3. I have no regrets, if GF really didn't want us to get those Pokémon, they shouldn't have made slots for them in the National Dex in the first place.
I used cloning in Emerald for two things: replicate my Gen 3 Living Pokédex so I could have one Living Dex per generation, that saved me a lot of time spent breeding, at least as long as Gen 1, 2 and 3 Pokémon were concerned; and clone a few Lucky Eggs to prepare a "curb stomp, anything goes" kind of run through some of Gen 3 and 4 games. I find joy in being overpowered :) ... but in practice, even getting Lucky Eggs for the entire team simply means that if I fight every single trainer in the game, I simply don't have to grind before the Elite Four. So, yeah, I get no actual advantage from this, it just saves time.
Speaking of runs with Experience-enhancing items, I don't do such things before completing the game normally at least once or twice, I just figure once I complete a game normally and meet all the challenges the designers put up, I can have some fun with it.
That's pretty much all I do. I don't care about EVs/IVs because I don't battle online, and the closest my cheating came to affecting people around me was that one time when I bred and put on Wonder Trade a few perfectly legal Phiones (you could even say "half-legit" since the Phiones were all OK, it was the parent Manaphy, who, while legal and not at all outstanding in terms of nature or IV, did have dubious origins :) ).