If the trench is to run up/down the slope, the recommended direction is to dig with the backhoe from the top downwards. The tractor will be facing downhill, and the tendency of the backhoe to pull the tractor will be countered by gravity. A possible problem is if the trench fills with water, then the downhill end (where you are digging) will fill. In this scenario it may be better to dig from the bottom up. If you do this, putting weighting in the FEL is suggested to balance the digging action at the rear end.