Some good comments on here but, not all address everything that will be an issue with static electricity and fuel. Most have it correct that you want to "bleed" off the electrical charge built up on the can before filling. Typically this is more of a concern with the cans on the outside of the vehicle due to the air passing over the can causing an electrical charge to build on the can.
Second, an additional charge is built upon the can when you are filling the can with fuel. You should have the metal hose in firm contact with the can (metal or plastic) and/or the can on the ground or grounded. The more surface area you have in contact with metal and the gas can, the better. The more ungrounded surface area on the can, the greater the charge that can build as plastic is obviously an insulator. A charge will build anytime you have a material moving through a plastic. Think about wood dust as an example. Anyone who has ever connected a shop vac to a belt sander has experienced a pretty darn good jolt when contacting the hose after running awhile.
Wind is your friend any day that you are filling these up. This is because your are diluting the gas fumes but, you also distribute them. All Flammable liquids have an LFL (lower flammability limit) and an UFL (upper flammability limit) and, as should be obvious, this is where the concentration of the material with the in air, create the flammable range of the material. This is why repair garages, as an example, are required to be ventilated to help reduce the possibility of creating a hazard. The National Fire Protection Agency also recognizes ventilation as a way to mitigate the inherent dangers and, in some cases, "declassify" spaces based upon the amount of air movement when properly designed. Gasoline has a range of roughly 1.4% and 7.6% depending on the temperature of material and air. Pretty low so here you see the hazard.
Keeping your cans inside your vehicle will not allow for natural ventilation to assist in distributing the vapor and, to make maters worse, they are now contained in the vehicle and the fumes can "pool" in pockets that temporarily trap the material as the fumes are heavier than air. Another example, in a repair garage, the space from the floor up to 18" is considered a "classified" location by the NFPA and as a result, special precautions are required when designing the electrical systems that enter or pass through this area.
Static and vehicles are ever present, inside and outside as traveling through the wind will "charge" the vehicle. You discharge this when exiting the vehicle and touching the earth and skin of the car at the same time. They make "ground straps" on vehicles that you can attach to your axle to help discharge the static when stopped. These are strips that will lift off the ground at speed and come back into contact with the ground at lower speeds or stopping. This will drain the energy harmlessly to ground before exiting the vehicle. Honda specs tires that actually have compounds to help dissipate this charge as well!
To illustrate how big a deal static is. A number of years ago there was an explosion involving gasoline and a fuel delivery vehicle. All standard practices were used for the transfer of fluids, grounds were attached and all precautions followed. The ignition source was found to be a steal hatch on top of the vehicle that was open. The lid was no longer in good contact with the body of the fuel tank due to rust build up on the hinge of the lid. This allowed a static charge to build on the lid, and this is the interesting part, a cloud passing overhead imparted a charge to the lid. The energy reached a limit that it caused an arc when it could finally bridge the insulation (the rust). The lid and the fuel tank were a different voltage potentials and wanted to equalize causing the arc.
So, would I ever fill a fuel can inside a vehicle. NO, it is not grounded to the vehicle and you are containing the vapors inside creating additional hazards. Would I do so with the can outside the vehicle but not on the ground, yes. But it has to be in direct contact with the metal frame of the vehicle which must be then, in some manner, connected to the earth (remember paint and power coat ARE insulators). You are attempting to eliminate differences of potential. If there is no difference of potential, there is no possibility of an arc being created.
Sorry for the long post, my electrical roots are showing.
