Floats that pivot front to back will dance around under hard acceleration and hard braking. Floats that pivot side to side will dance around under hard cornering. Drag racers and oval track racers modify their carbs for that. For normal driving neither position is going to be perfect so mount the carb the way that provides you with the neatest and simplest fuel line and linkage hook ups. It will be fine for the street.
The only foolproof way out of the float problem is to run fuel injection.
