There have been many posts about this in various Tesla forums. The standard rule of not driving too fast applies all year, where “too fast” is generally above 65mph, which is when efficiency takes a significant hit from air resistance. Many people have determined that using the seat warmers is the most efficient way to warm the cabin (instead of using the air heater). While this has been proven to be true, what this really comes down to is what’s your comfort level. There are enough Superchargers around (typically spaced 100-150 miles apart) that you should not have to worry about actually running out of battery, but you should plan ahead and assume cold weather trips may take a bit longer.