Ahh yes. Rebooting is the safer way to go to be sure.

By not rebooting and coasing with everything dead your mileage at the pump might be a little higher cause you have less load. The displayed mileage should be closer to the real world, and your OD will have a few percent less miles on the total. For any normal sane person giving up the comfort and safety just to squeeze a couple tenths out of a car would be crazy. I've never claimed to be sane and I'm only 2 tenths out of the number one spot in the database for HCHs so I may start givin it a try.

Edit:

I'm so dumb. Whatever slight increase you might get from having the power off the car is not going to record. You would not get credit for any miles driven this way.