If you remember two things, you'll be all set...
- When the car is first driven, don't accelerate harshly (you probably don't anyway, if you're interested in getting the best FE out of the car, and I see a 49mpg average, so I know you're doing well in that regard). You won't harm the car, as long as you're not doing drag runs with it out of the box.
- The car is getting 0 mpg anytime it's idling and not moving (and dropping down your tank average, as you noted).
The car will warm up faster if you drive it (again, not TOO harshly), and you'll have used that gasoline for something besides emitting hydrocarbons into the air
That being said, I'll usually let the car warm up for 5-10 seconds in the *really* cold weather. Otherwise, I'm usually shifting into gear and starting to move after I fasten seat belt and reset my Trip 'B' (I use that for my 'per-trip' mileage, and 'A' for the 'tank avg') - that takes about 5 seconds, I think.
Generally, I use that procedure most of the time I get in the car. Occasionally, as I'm out / about, I think to fasten the seatbelt *before* starting the car, and, on the weekend, I'll leave Trip 'B' in place for the day and not reset it each time I start / drive the car from trip to trip.