I just aim to get "good" IVs. 23+ in the stats that matter, but more if I can't make up for the losses with EVs. Like if I wanted a max speed electivire, I would keep breeding until I got both a 31 in spd, and 23+ in Atk, HP, and SDef (this thing is going to be switching into electric types a lot, which are usually special). Or if I needed to be able to hit a certain stat to survive or kill something 100% of the time, I would continue to breed until I got the stat I needed for that, too, whether it just be 24, 27, or a perfect 31. It depends a lot on both max stats and spare EVs.
edit: there's also going for a good Hidden Power on pokes, which is a complete and utter pain. I usually accept slightly lower IVs in the less essential (but still important) stats. Again, except in the case when I need to be able to hit a certain stat.