Are there optimal positions for A/C and heat register grills to be facing?
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.