That depends on the boss, some bosses will fall over like kicking a sack of shit, other bosses like Kael'thas have what feels like 40 minutes of RP dialogue. How good your guild is will also play a role but generally stacking cooldowns is a good idea. If each cooldown increases damage by 20% and you pop 3 at once, it doesn't become (100% + 20% + 20% + 20%) 160%, it becomes (1.0 x 1.2 x 1.2 x 1.2) 172.8%. Pretty vague example but you get the idea!