If you sit facing the way they come from, it could help you avoid being spotted because if you see them before they see you, you can avoid moving and being seen and can be more prepared for the shot. But, if you sit facing away from the direction they come, the tree trunk will help hide your body and movements. There are reasons to do both, depends on situation. What else is in the area such as feeding trees, creeks to cross, blowdowns, wind direction etc. There is no perfect answer, every set up will be different.