I find rewarming can be necessary, typically you can tell by the sound of your sled (slower at picking up to idle rpm). Basically, I know a good temp to finally give it to her is within 20*F of the ideal cool riding temp (if you have a temperature gauge, it won't take long to learn what temperature your sled wants to ride at to give the best response and gas mileage).
When first taking my sled out: inspect suspension and track, freeing up any ice that might rub or interfere with the performance of the sled. Bounce the arse end of 'er 3 times to help aid in this process

. Make sure skis aren't stuck (if it's on snow, you should be fine, however I've had my skags stick to PAVEMENT and was kinda shocked by the whole experience). Start the engine. If it's cold, it will not idle. It will stall out. Start it again. If it's really cold, it will do this again. Start it again, it should be fine now. After it has idled to 35 or 40*F, gently burp the throttle to prevent oil build up during the warm-up process (not necessary, doesn't typically happen, but I find if you let any 2-stroke idle too long, like say 5-10mins, you'll cut the spark plugs life in half) and do so for each increase of 5*F until it is within 20*F of your ideal cool riding temperature. Then, by the time I cross the street, ride by 4 houses and a building, and hit the trails, it is above the ideal cool riding temp and I can GIVE IT TO HER! Because if I don't let it warm up that much, I know I will not be able to refrain from giving it a huge burst of gas the second I think it's safe to do so.
I never HAVE to do the warm up like this, but may as well. Why not? lol.