There are millions of cars filled with gasoline every day around the World yet you don't hear of a single incident of fire attributed to static discharge. I suspect that with the similarly small quantities of AVGAS we put in our 'planes that we have a similarly small risk. I think if you're filling up a DC6 or similar then the quantities concerned might generate sufficient static.
However, we still stick rigorously to the routine of attaching the static line to our PA28 and C172s when filling up at the pump.
Now, our friends who fly microlights upend their Jerry cans of gasoline and merrily glug the contents into their aircraft without even a thought about static bonding!
TOO