Altered Time - your #1 World of Warcraft Mage resource!

So usually in the rune after combust do you fireblast without heating up to keep Blaster master 3 going or do you save it until crit to get Pyro. And is there an exception if you cleave many adds e.g Hivemind where mastery might be more valuable?

I find myself often going for the blaster master refresh hoping that in {crit%} of cases i will get the pyro anyway.