Hi!
It's not a myth! Tell any pylon racer(or any one that has been around this hobby for more than 20 years) and he will laught at you!!!

The tank must always be mounted so that the center of the tank is inline with center of the carb (or more precisely the carb intake orifice) when the airplane sits horizontal. This is something all experienced fliers know!

The reason for this is the fuel pressue alters with hight! Simple as that ,and all our engines react on altering fuel pressures,some more some less.