In theory it doesn't make any difference to the tank venting, since it is just sucking air in through the valve in the cap, and bubbles just rise through the fuel instead of entering above the fuel. If you suspect it is a tank venting problem, just try it the right way up, with the fuel cap loose. If it runs better that way than it does upside down with the fuel cap tight, you might try inspecting the air valve in the cap. I've had them stick due to oxidised oil, when the machine is out of service for a long time. If the valve is stuck just wash it with petrol, holding the valve open while you do it.
