Actually, the preflight maneuver was to spin them clockwise 360 degs.
The reason you did that was to check for coke-ing of the turbine.
Older C20-s had a problem with this, and by spinning the blades
backwards you would also be spinning the turbine and you can hear
and feel it. I've never heard of the opposite rotation being done,
but im sure someone else will come up with a reason that it was.
btw: spinning the turbine is still part of the postflight of the
MD500, I was told that its because of the crazy engine angle.

The reason for the 90 / 270 thing was to ensure that they were not
still tied to the tailboom. One of the funniest/scary stories I heard
was that during a maintenance run-up the pilot checked for the 90/270
blades, fired up the turbine, and then noticed (too late) that the
mechanic had secured the blades to one of those large rolling toolboxes.
The box rolled around and did some serious damage before the pilot
could stop it. No one was hurt that I know of though.

Bart

Jim Carriere wrote:
I remember part of the Bell Jetranger preflight was to turn the blades by
hand to 90/270, and you were supposed to turn them counter clockwise (the
direction of normal rotation), I can't remember the reason why that
particular direction was specified. Maybe so as to not spin the power
turbine (and engine RGB) without proper lubrication, maybe it was to check
for sprag clutch disengagement, maybe both reasons.