The trick is to mount the directional tyres all the same direction. If
you put a rear pointing tyre on the rigth rear, and a front pointing
tyre on the left rear, you are likely to loose your rear axel as the
two tyres simply turn the axel around and around  as the car trys to
go down the street. Also this is very bad for gas milage since the
rear tyres are fighting the front tyres and it is not very good for
tyre milage either. Same situation and considerations hold for tires
also. Now if you mount both rear tyres backwards, then you are likely
to stretch the car into a stretch limo, but be carefull as cars have
stretched too far and have broken this way. If you mount all tires
rearwards, not to worry, You just drive backwards and all is fine.
Proper use of the spare tyre has never been solved properly. It has
been suggested that the whole concept of directional tyres is too
complex and tyres should only be made to rotate either way. In fact,
rumor has it that the new Democratic liberal senate and congress will
take this up as one of its first items, shortly after it bans
ammunition for guns.

The tread is directional, the internal construction is not. You'll get
less traction running it backwards, but other than that you should be
fine. I would guess that it would probably wear faster, too. I would
also be careful in the rain, as the tread will pump water under the
treads instead of away from the tread.