I noticed a while ago that some of my A/C and heat register grills are facing the walls and not the room (the insides of the curves on the grills face the walls) in some rooms but not in others. My assumption was that they should all point towards the room, not the wall, that some of them were put on backwards. Is that a correct assumption or is there some reason why the system would benefit from grills pointing at walls, like for airflow reasons or something? Or does it not matter at all, but is a preference thing to not feel the air blow on you?
You want your vent venting to air as much as possible. If you are cooling/heating your walls/floors/ceiling this is much more inefficient than doing the same to the room. Think about it this way... Let's say 25% of the heat is absorbed by the wall. Do you think the wall will heat up the room to reflect that 25%? Maybe reflecting 1/5 of that if that.
The other factor is that heat rises. So generally you don't want to be heating the ceiling. This isn't always easy to "fix" though. General rule is vent to air and make sure vents areas don't have obstructions for air flow.
What a great question. I love questions that hit the sweet spot between a textbook, and the real world.
Textbook Physics says that you save energy by pointing the vent to blow to the room.
Real world physics says that blowing toward you better when the A/C is on, and to the wall when heating.
Personal preference is, of course, a personal decision.
In pratical terms, the personal decision is the right answer, I personally think, unless my wife does not agree with me.