I pay by the gallon. I rarely have a completely empty tank to fill. The company that fills my 20# tanks will weigh them and fill them by the metered gallon. More than once I have had 3 tanks filled and they have all been on the same ticket (NM law prohibit transporting more than 3 LP tanks in a passenger vehicle at one time).
My choice of where to fill is dictated by the fact that I know the steps taken to correctly fill a tank. I've walked away from a fill station, with my empty tank, when a step is skipped.
I carry two 20# tank on the truck camper that I refill. One is "mine" (painted light turquoise) and the other is painted white. The white one is my "swap" tank. I always carry a tank dedicated for swapping. As that tank nears it's exp date, I replace it. I do not replace my other tanks. I take very good care of my tanks. They are rust free, dent free and clean. I repaint them every few years as they get skint up from use. When I refill, the tanks are inspected, particularly the foot ring, before refilling.
The people who fill my little tanks work for and are trained by the same company who fills my big horizontal tank.
You have to be careful when swapping tanks. More than once I have gotten an out of date/recertified tank. My last 4 tanks, I ended up buying new. It was just easier to do it that way. It has gotten hard to get a newish tank from the swaps in my area. I have rejected several "swaps" due to condition too.