The shock guys will tell you you get maximum efficiency with the straight up and down configuration. There are charts I've seen that tell you how much "degradation" you get per angle of inclination from vertical. That being said your visualization of the axle swinging on an arc to one side or the other is accurate, and speaks to why an angle mount works well enough. If you were going drag only I'd say vertical because you'd be unlikely to have uneven axle deflection when needed most (or hopefully). If you think it will go street at some time and don't want to change it then, some inclination now wouldn't hurt that much.