Focusing just on temperature isn't the way to tune, regardless of ambient outside temps. As long as you have good smoke throughout the throttle range with no hesitation or bogging (too rich) you should be fine. On the flip side (too lean), if your engine drops the RPMs slowly after letting off the gas you are too lean. The RPM should pretty much closely match your trigger input. A quick blip to WOT and back should go rrrrrRRRRReeerrr... not rrrrRRRRRRREEEEEEeeeeerrrrrrrrrr. That "hang" of the RPM's indicates a too lean setting.
There are 2 things that will kill an engine dead: Lack of lubrication and excessive heat. A visible smoke trail indicates you have enough fuel in your mix that there is sufficient lubrication. Excessive heat depends on the mill but anything over 300 is usually a bad thing due to the stress on the moving parts.
The reasons people suggest wrapping your head with foil or a sock during cold weather runs is to get more meaningful average temperature readings but also to reduce the amount of re-tuning needed. With an unwrapped head in cold weather, the metal cools faster than normal. By the time you get your temp reading it'll already be off a bit. The sock/foil helps to mitigate the faster heat loss of cold ambient temps so when you take your reading you're closer to the "real" temp. To illustrate this, try the following: while outside when cold with the car up on blocks and no wrap on the head, run the engine at high RPM for a while with your temp gauge taking constant readings. Then drop it down to idle, again taking constant temp readings. Notice how fast the temps will fall. Do the same test with the head wrapped and you'll see that the temps fall more slowly. You'll also see that the idle temp wrapped vs unwrapped is higher.
Engine temps are a guideline, not a rule. As long as your peak temps stay below a reasonable range (less than 300), above operating temp (somewhere around 210), visible smoke trails, and you have good performance, you'll be fine.
