Try calculating the same thing on fights that ever require you to move for ~10 seconds. You don't need to be forced to move too often to make more than 2/5 look like more than just a neglicible increase. And if you need to reapply from 0 (say if you stopped for 15s or more due to how the fight works, for example), anything less than 5/5 is most likely unacceptable.
At the end, it really depends on the fight. But might as well spec for all fights - if you're making a pure DPS spec getting up there:
http://www.wowhead.com/?talent=of0Vf0cZZVAMcof0oqqt
All you lose is a point in ice flows, and you could still go 4/5 and get 2/2 ice flows.