I typically don't go oom in 25 mans with all the mana regen buffs/debuffs around, so dragonhawk all the time, but if I do go oom, whcih happens on fights such as 10m deathbringer with all the add switching, switch to viper when you have as few procs up as possible and switch to dragonhawk when you have to chimera and aimed shot then just switch back to viper if you still need more mana. Try to make sure you aren't in viper when you're using aimed and chimera, just do a quick switch for those two shots.