You'll definitely want it at a good operating temp before hard running, regardless of the ambient air temp. Even when its hot outside, the engine will need to warm-up before you run it hard, but in any weather (hopefully dry) you can usually accomplish this by running it in smallish, controlled circles.

A word of caution, do NOT let it just sit and idle for extended periods of time (regardless of how hot or cold it is outside). If the engine just sits at idle, it will probably not reach operating temp and you WILL wear down the piston/sleeve taper seal, and then, when it DOES get to temp you will have very little compression.

There is a little more to that (how it seals at various temps and how the metals expand when hot, how the tapered parts work, etc) but just remember NEVER let it idle for a long time, keep some heat in it!