I posted this elsewhere where this question was also asked:
Don't worry about it. I discussed the properties of gasoline in prior discussions here, which make it nearly impossible for the flame at the back of your fridge to be a danger at a gas station.
Gasoline fumes are heavier than air, they stick low to the ground. The fridge is 2-3 feet off of the ground, well away from any fumes. Plus, those gasoline fumes will be mixed heavily with the surrounding air, which brings up the next point. Gasoline has a very narrow flammability limit. Just a little too much air and it won't ignite. Finally, gasoline fumes are controlled before they even escape. For years many states required vapor recovery systems at fueling stations. The fuel pumps had the collapsible rubber "boots" which pressed against your car while in place. This sucked the vapors into a filter while the fuel entered the tank. This is no longer required in many states since onboard fuel vapor recovery systems have been on cars since 2000 and light trucks since 2006, so since a high percentage of cars on the road now have this, some states have eliminated the requirement at fuel stations.
If you want to turn your fridge off, it is the safest option of course. But necessary, no.