we extended when we reached the 3 last bosses in ToT, to get them all down, but somehow we didn't need it in SoO.
We only raid a few hours pr week so the long raids gives a rather limited time to progress on a new boss, which often limits the progress. It's a bad idea in the beginning unless you're good enough to clear it all so you can go HC next week, since you miss out on the gear.
Later on extending is fine though. A bit of gear shouldn't change it since it's out geared already. More about getting mechanics right.