When your tank is full, there is less chance that it will pick up moisture from the air because of temperature changes. I am not fanatical about it, but I try to keep a full tank at all times unless I know that tomorrow I will be using the tractor again. If I don't see any immediate reason to use the tractor I will fill it, and it will usually take a gallon or more. I usually top off my generators fuel tank every month because it will run for a few hours once we have a power outage, and I don't want to risk getting caught without fuel if we have another 10 - 14 day power outage as a result of a blizzard like we did about 10 years ago. Ten days before power was restored, and 4 days later, the power went out again, this time for just shy of 14 days. That is when I decided that we needed a full-house generator. Hasn't gone out that long ever since, but you never know when the next big outage will come.