William Sommerwerck wrote:
Mythbusters, anyone?

To the best of my knowledge, there is nothing in any motor that
//mechanically// favors rotation in one direction. "Forwards" and "backwards"
are determined electromagnetically, not mechanically.

The maximum speed before the bearings are damaged depends on the bearing
quality, I assume. The only damage I can image is knocking the shaft
off-kilter.

well for brushed motors, there can be mechanical differences. Brushed
motors can be "timed" to alter speed/torque as well as to support
bidirectional use when the brushes are set to a neutral position. This is
determined by the angles between the brushes and the magnetic poles in the
motor. The same applies to universal motors as in power tools and such. As
you don't run a circular saw or angle grinder backwards, you can bet that
motor was designed for maximum performance in the direction it does run.
Maximum performance can also just mean getting by with the smallest
cheapest motor possible to get the job done.