I came on this:
The teeth on Auriou rasps are slightly angled to favor right-handed users. If you are left-handed, please contact us as we can order you left-handed rasp (although we do keep some in stock). The reason has to do with the proper use of a rasp.
A rasp should be grasped by the handle in one hand. This is where the power behind the rasp comes from. The other hand should grasp the tip of the rasp in thumb and forefinger, to balance the pressure on the stroke and to give control. The teeth should be perpendicular to the direction of the stoke. Auriou rasps are handed, so you hold them at an angle to the work but push them perpendicularly to the work.
I understand what they're saying. My question is, how much does it matter? If you're using such a rasp 'off-handed', is it noticeably less effective?