I've always driven tiller steared boats sitting on the starboard side of the boat. for one (as others have pointed out) the gears are on the starboard side of the engine, and I feel you have greater control over the engine. I always teach that you sit on the starboard side too.
Mind you, there are always times when sitting on the port side are advantagous, though usually in a rescue situation (i.e. towing a boat alongside while capsised - was doing this today
) - though after many years of driving using both, it still feels very wierd driving on the port side when I do it