As has been mentioned, the only thing that is important is the vertical relationship of the tank to the carburetor. Other than that, you can rotate the tank and the engine independently of each other to suit the application. For example, Sullivan tanks have a bubble on top, but often I have to install these tanks sideways because the tanks are wider than they are tall and sometimes the fuselage is too narrow.

Of course if you do rotate the tank, you have to rotate the vent tube so that it points up (inside the tank) or you won't be able to fill the tank all the way.