Just a little above 9k buffed is a good number for any bosses which requires some hp - with the exception of Archimonde as he is just a battle of survival.

Of course the more the better, however, do not gimp it for your key stats - crit/dmg or whatever. Yet, if you can down Kael and Vashj, the first 4 bosses in BT and Hyjal should be cake and you will get your decent gears with good stats from then on. T5 gears and equivalent - with the exception of tanks - will be good enough to clear the whole of BT with exception of Mother which requires some SR gears. FRs too for Illidan, but that's not obtained in T6 content raids.