Unless you top off the tank every time, which for some reason is not recommended, there can be quite a big difference in how full your tank is.
What mileage you are getting isn't as important as seeing it drop, either suddenly, or over time. I say if you want to check your mileage, pick a method and stick to it.
I use the EVIC and take a look at it every couple of months. If it varies from tankful to tankful, so what. Unless you plan to tear your engine apart to look for that extra half mile per gallon, mileage is like pro football scores. You have nothing to do with them, can't change them, but can talk about them into hell freezes over.