Since Emerald, I've been semi-casual. I only train what I like, though I will EV train (since it's so darn easy now) and put some emphasis on natures so they can be used in Battle Subway, PWT, and White Forest. I don't stress IVs, but I do have some Dittos that I will use to pass on an appropriate 30/31 IV via breeding.
I always do that in-game. If I am battling a friend I either use my in-game team or I breed new Pokemon and only try for certain natures. I only bother with Evs and IVs in the Subway/Frontier-like areas. And WiFi of course.
I always play through with whatever I like, ignoring a lot of those 'extra' mechanics (I don't even look at natures. I mean, I look at it, out of curiosity, but more because I think it gives the monster more "personality;" I don't care if it's a bad nature or whatever).
After I've completed almost everything in the game, I'll start breeding for the IV/nature/etc and training with EV. But I only do this for stuff like Subway or playing in the VGC. If I don't feel like doing those things, I don't. --- http://reddragonranch.blogspot.com The Legend of Red Dragon Ranch - KICKSTARTER now LIVE!!