patzane,

The reason is for proper fuel flow. Whatever the orientation of the stopper(fore or aft), the fuel pickup(clunk) needs to be at the rear or towards the carb of the engine.

Forward velocity pushes the fuel to the rear during foward flight so that's where the pickup must also be.

Both placements work and it's usually whatever the individual favors the most. I prefer to have the stopper pointing towards the radio compartment and running the lines back over the tank. I resist placing bends within the tank itself. Too hard to get at to make any "adjustments", LOL...

PM