I always install a plug before transporting the airplane in my trailer. I like to keep the fuel smell at a low level. I always keep the airplane plugged in the shop to prevent the smell of Jet -A from stinking up the shop.
As you turn the airplane over, a small amount of fuel will spill out of the main tanks through the vent. There will sometimes be a small amount of fuel in the mains, even after defuelling.
I use a start and taxi tank during fueling, so the plug must be removed before hooking up the S&T tank. I use an old RAM fuel pump on my fuel can, on 7.2 volts to keep the pressure down.