Tires is another aspect of tuning. It is separate from everything else. One thing at a time. There are many things that affect handling.

In regards to nitro/gas cars, I'd use the average or middle fuel level. The worst case scenario is that your imbalance is off by half the weight of the tank. If you tune to either full or empty, the worst scenario is that the weight of the entire tank is the amount you are off. Location of tank will affect things too. I have only ever run electric. Keep in mind that on a real car, if the vehicle weighs 2500 lbs and a tank of gas is 18 gallons, that's only a little over 100 lbs difference full to empty which is only a few percent.