First off, I wouldn't swap Permafrost for Improved Blizzard, I generally take both. Something along the lines of
http://www.wowprovider.com/?talent=1121 ... 33132051c1I'm looking at this from a peeling/CC perspective, because when I play a Mage in PvP it's never the damage that concerns me. Even a green geared Mage can be more useful than most other classes if played well.
For this reason I practically never use max rank Blizzard in PvP, the rank one slows just as much and has a very low mana cost (272 mana).
This goes for Cone of Cold as well, to some extend, as when peeling I'll utilize the rank 1 to preserve mana. I only use the max rank if I want to burst down a target attacking me.
I would have given you a great example of a mage using Blizzard to CC the opposing team to great effect, but it appears to have been deleted from youtube.
Regarding PvE, you have to realise that with 3+ targets, the chance for Blizzard to proc clearcasting increases dramatically. Even if your manapool itself would be so low that it would not allow a max rank Blizzard, you can always start out with a rank 1 Blizzard and then swap to full rank 1 or 2 ticks later when the clearcasting procs.