Is there any issues at all with having cans that are not full? Not sure if the sloshing around is any impact on the relative safety? Also what about the summer time? Are there additional concerns for the heat in some areas that the cans are exposed to?
It would seem that a good policy for safety and fresh fuel, may be to fill the cans prior to a trip and upon the return empty the remaining fuel back into the tank.
Also, is there an advantage or disadvantage to keeping the fuel, and the weight on the back of the rig while off road, versus moving that weight into the tank as soon as there is space?