If you use Icy Veins and Combustion at the same time then Combustion will go back on cooldown faster but I'm not sure if that outweighs holding out until it is up again to use it with Icy Veins. I don't think it would make any difference expect in a vacuum, anyways.

Waiting for icy veins to pop combustion just so that your combustion comes back up faster is quite counter-productive.

Nope. Using Combustion + Trinket + Icy Veins maximizes 1) your crit chance during the effects of Trinket +dmg 2) the number of Fireballs you get off during Trinket +dmg regardless of crit chance.

If you have a trinket obviously you'll be waiting on combustion for the trinket and then on the trinket for icy veins, thus effectively delaying combustion. This doesn't mean that you should just delay combustion for icy veins every time. If I say A is a better thing to do in X situation, you saying B is better in Y situation doesn't really contradict is. Bottom line is there is no point waiting to pair icy veins with combustion for itself, although you may end up finding yourself doing it if you're trying to pair each of them with the trinket.

Also notice that not all cooldowns are worth waiting for. For example in a situation where the only cooldowns that are almost ready are trinket and icy veins, if they're close enough you should wait on the trinket for icy veins, but you should never wait on icy veins for the trinket no matter how little time you'll need to wait, at least according to some rough math I was doing.

Would be nice to get some general spreadsheet (or include in vontre's) what are the breakpoints for certain cooldow stackings to be worth it over seperate use. As obviously waiting 5 secs on the trinket to use icy veins is worth it, but waiting 1 minute is not. All assuming you're not sure how much time is left on the fight, of course, as if you know how much time is left you could base your decision on "will this result in 1 less use if I wait?" and if you answer yes, pop it now, and if the answer is no then wait.

Right now, there is very very few evidence that has been shown to truly display the effect of cooldown stacking versus not doing it. I have a very rough idea, but nothing worthy of TC.

I will absolutely agree with you here. For PvE sustained DPS there should be no significant difference between using Icy Veins and Combustion separately or together. For PvE or PvP burst DPS of course the matter changes, but PvE burst DPS is often unnecessary. When actual power increase cooldowns are used, whether they're trinkets or other gear-based procs or battlegrounds Berserk buffs or Power Infusion or Flame Caps or Destruction Potions (but not Bloodlust per se) is when you get a chance to significantly skew your average damage per hit by using Combustion and Icy Veins and Berserking.

What about cooldown management for arcane/frost build like 40/0/21? I tend to think to do the following:

1. Warm up with Arcane Blast to proc the Blessing deck to +80 dmg,
2. Pop Arcane Power + Icon and Spam Arcane Blast
3. Pop Ivy and spam Frostbolt (since pop Ivy with Arcane Blast spam runs into GCD issue).
4. Pop Cold Snap and repeat 3 again.
5. Resume normal rotation with ABx3, Fbx3 when cooldown is not up.

Does this work? Should I wait on Icon (2 m) for it to stack up with Arcane Power (3m) again and repeat 1-5? Or should I just pop it when it's up during my normal rotation?

I know that the common practice is to use a mana gem/mana pot as soon as you can, but what if you timed the start of your Icy Veins so that after 8 fireballs with the Icy Veins buff, your mana pool would be 40% (or 20% for the 2pc T6 bonus), and then use the remaining 2 seconds of the Icy Veins buff to channel Evocation back to full mana. Would this be a bad use of Evocation? Is the 20% time that you save on your Evocation worth more than that extra Fireball that could've gone off in that 2 second time frame, or is it better to just mana gem/mana pot as normal and Evocate later on?

If you must Evocate at some point, that is not a bad use of Icy Veins, as the saved time will more than make up for the Fireball you could have cast. In essence, you're getting well over 20 seconds of hasted time by using Evo right at the end.

You don't need a full two seconds left on IV to start to Evocate, though. You only need enough time that you're actually channeling before IV fades.