Im no motor tuner, but ive heard this information on a few occasions and led to believe its no bs, the way to think of it is although the brush hoods maybe 90degs to the endbell screws, the brush hoods themselves are not always the same dimensions as they can open out through use, or poor manufacturing tolerances....
so the brush can tilt ever so slightly and run off centre to the commutator.
Less brush contact can give all sorts of problems when its not running flush to the commutator, such as uneven brush wear and increased amp draw.
Fusion Hobbies / www.horshamrc.org
The wife stops me being sane and the r/c stops me from going insane....