Edit: cars with dual exhaust of course don't have a choice--but most cars don't seem designed for dual exhaust from the get-go. [How many with duals are still a single until the rear bumper?]
My car(Lincoln LS) is 100% dual from the headers to the exhaust tips, without even a crossover pipe in the mix. As far as I know, all variants of the car-both V6 and V8-are set up this way. The filler is on the passenger side.
The MGB had it's filler coming out from just under the trunk lid on the passengers side while the old GMs had their fillers under the license plate.
I usually fill my MGB from the passenger side, but one of the nice things about the arrangement is that-combined with the narrow width of the car-it's not a big deal to fill it from either side.
One modification I've seen on a couple of cars recently is to move the fuel filler inside the trunk(or should I say boot?). It's technically not that difficult to do, as there's a big rubber hose running from the filler pipe to the top of the gas tank that's easily seen inside the trunk. I don't quite understand it, as the fuel filler on the outside(especially with a chrome cap on a chrome bumper car) doesn't look that bad. I always manage to spill a few drops of a gas when I'm filling the car, and I'd rather it land on chrome of the bumper than inside my trunk.
BTW, the exhaust on the MG DOES run down the driver side, while all the fuel related stuff(pump, lines, etc) runs down the passenger side. Of course, with the carburetors on the driver's side, the fuel line still has to make a cross over, and it happens somewhere in the region of the transmission bell housing(I think). The fuel line still ends up somewhere in the neighborhood of hot engine parts.