Been doing some Firelands runs with the 4pc T13 as Frost (and I'm already getting tells trying to convince me otherwise). The 4pc bonus, unlike Theorycrafting, is not going to give you a 24 second cooldown on Icy Veins every time no matter what you do; you are guaranteed a 24 sec. Icy Veins once, and once only, because the second time you pop it you will not have enough stacks of Stolen Time to bring it that low by the time it's up.
In a sense, the 4 pc T13 bonus is more akin to popping Cold Snap, except you have an actual Cold Snap when you find need to carry it past two uses.
There are other threads that go into more detail about the 4P bonus, but suffice it to say that to use it to maximal effect you need a windup. But should you go for ten stacks?
The choice is obvious for arcane (yes), given the consistency of their stacks. Similarly, the choice is obvious for fire (no), given how random fishing for DoTs can be. But for frost, the answer is also no - for weirder reasons.
See, the 2P bonus is a minor bonus on its own - where you really can get results is in the shorter cooldowns on Icy Veins. Thus we're looking to minimize that cooldown with just the right number of stacks; its cooldown should be equal to your windup time, plus its duration.
Assume for the sake of simplicity that the vast majority of your casts are Frostbolts, which can trigger the proc. (Some casts will be other things, too, like Frostfire Bolt/Deep Freeze/Ice Lance, but let's stick to a basic model.) Further assume that every second Frostbolt triggers the proc (it has a half-chance of triggering for each cast). Finally assume that the Stolen Time stacks do not significantly alter your Frostbolt cast times. Thus, assuming a net Icy Veins cooldown equal to its duration plus your Frostbolt windup, we have the following equation:
(C = Frostbolt cast time, S = Stolen Time stacks when Icy Veins is cast)
(Cooldown) 144 - 12S = 20 + 2CS (Duration + windup)
124 = 2S * (6 + C)
62 = S * (6 + C)
62 / (6 + C) = S
This is the ideal number of stacks you'll want to shoot for. In my current gear/reforging setup (not ideal for Frost, of course, too little crit) my Frostbolts have a cast time of 1.68 seconds. Thus my ideal number of stacks (S) would be 62/7.68 = (approximately) 8.07.
To be fair, this model is imprecise. Stacks build up at random, and the frost rotation contains some spells that don't actually build stacks. On the other hand, Frostfire Bolt (when used properly) can build stacks at a greater rate (being instant-cast), and Early Frost will shorten one Frostbolt cast time every fifteen seconds.