As above, empty forks should face down the slope, this puts the heaviest part of the truck behind the braking wheels when travelling down.

Reversing down a slope and braking in an emergency would cause a similar effect to performing an emergency stop using the parking brake in a car, the wheels will lock and the counterweight will drag the locked wheels down the slope, this will lead to a longer stopping distance and more importantly could cause the truck to slide sideways on the slope, and overturn when it meets the level ground at the bottom!!

Travelling uphill with forks leading can also lead to wheel-spin and loss of control, visibility should not be a problem when reversing, there are no blind spots caused by the mast uprights when reversing so visibility is actually better if the operator turns in the seat and looks in the direction of travel as per their training.

Hope this helps