That depends on what sort of a power source you are using. It is always best to get the highest N1 possible before throwing the fuel to any PT6 to make for a cooler start. If it's really cold (like -20 to -40) and you are using battery start you will likely put the fuel on as soon as you get min N1 since your battery is taking an awful pounding and you won't likely get much more anyways.
Using a good warm battery, let it get to the point where the acceleration of the N1 is slowing a bit then give her-- especially if you are going to have to cross start the other engine. This is of course not applicable to the PC12!
With a GPU let it more or less stabilize every time (but not for long as you are burning the starter motor) before adding fuel.
I've done the "extreme cold" thing with a few of the PT6 series (including the 67) and always been told to go with the highest N1 possible respecting power available. Never had any problem getting the -67 to go with a good battery. The coldest I ever did was a -113a (Caravan) with oil temp down around -40. We did make min Ng but only just!
