For obtaining the mythicals- Pokegenning and exploits are fair game. It's not fair if you miss out due to a) distance b) time frame.
Legends- Well, I think if you're serious about competitive legends, with the full 6IVs, helpful natures and stuff, I don't particularly care. I don't believe they're used all that often- legends to me seem to be more trophies that you've completed X game nowadays than actual ones I battle with. If I can be bothered/remember, then soft resetting for nature is acceptable too, but most of mine are just catch as I see them.
One thing you didn't mention was cloning. I'm completely OK with it, having exploited the bug to hell and back in gen II :) Also a handy way of making large giveaways possible, which is no bad thing.
Obviously I have mad respect for those who have resetted for natures/IVs (easier now in gen VI), and indeed shinyness. I'd place more value on a shiny legend with a crap nature and IVs than one that is shiny and perfect and has all the right IVs, as one is rather obviously hacked, while the other is either the gods smiling on someone or dedicated soft resetting.
When it comes to normal mons, I feel that on the one hand, it takes up a ton more time to breed everything correctly, and if you've bred yourself a lovely 5IV physical Salamance but decide you need a special attacking one instead having to start from a fair number of eggs back before you get a special one is daunting and annoying. Also if you're doing regular competitions I can understand you not bothering, as you don't have as much time in between each one to get it ready. You need it now, I get that. As long as it's all legal movesets and details then I can't really look down on any of it.
However- if there's a legend that is in OU or below (say, Land-T), then you can't breed for IVs/Natures, only soft reset. This is when those who pokegen a perfect one are at a clear advantage to those who don't (and therefore have to sink hours and hours trying to get a good one), so that's annoying. Doubly so if it's a mythic like Keldeo, a very useful OU-standard pokemon who sees frequent use but completely unobtainable to the vast majority of those who can't pokegen. This places them at a clear disadvantage in terms of team selection and it's not really their fault. Thankfully regular giveaways of the thing do even it up somewhat.
Second, it feels more satisfying when it's ones you've legit brought up.
Third, though it is quite happily possible to get 5IV pokemon nowadays thanks to breeding mechanics, a pokegenned one will 9 times out of 10 be 6IV, which are distinctly harder to obtain through legit means (though not impossible). This is usually not a problem- a physical attacker is not going to use its precious 31 IV special attack, is it? But for mixed attacking especially I can see it being the tiniest of tiny advantages, as its 5IV counterpart will not be as good at it in some other fashion- speed, HP, defence, or special defence. If you were really dedicated you could breed enough eggs to get one that is indeed 6IV though (hoping you get the right ability, gender and nature in the process).
One thing I don't like though is shinifying everything. I mean, come on. They're supposed to be special! If you've
However, one more thing- breeding dittos. They're amazing, and the many giveaways of them are fantastic. Seems like everyone has a breeding ditto now, but they're using them to breed and raise legal pokemon, which I think is fine.
So yeah, if say I'd got an obviously hacked shiny 6IV charizard, I'd be a... little leery with battling with it, but slap an everstone on it, chuck it in the daycare with a breeding ditto holding the destiny knot and keep chucking out the eggs until I have a decent 5IV one and I'd be totally fine with it.