cabinfever, it's where the ring guides are that matters. if you take a look at a piston, there's the two little pins where the ring gaps go. it's not the same orientation if you turn it around.
on my engine, it actually looked like the ring gaps wouldn't be in the ports when i reversed it, but they were close. after reversing the piston, they had plenty of clearance.
like good vibrations said, it may be a crap shoot. it works great on my engine, but i haven't tried any other ones to see what the clearance is.
on another note, i have "fakied" my bike and bump started it backwards, but the engine still started forwards. weird. but i'm glad it did. i woulda hated to flip over the bars with my bike behind me...