Ultragauge: Do I need to reset AVG mpg at every fill up?

[sviking]

Well, I calibrated my Ultragauge over four tanks of fuel and the fifth tank was pretty spot on. Within .5 mpg of what the unit reported. Excellent! Now, as the title said, my question is:

Do I need to reset the AVG miles/gallon every time in addition to holding the top button to get the Ultragauge to recognize the fill up? I didn't do it when I filled up yesterday and the AVG mpg is holding steady and hasn't changed even a tenth of a mpg, which seems strange. Thanks!