I had to think about this one because it is something that I have seen hundreds of times but, I was not looking for it.
At first I kept thinking about the proper mounting of a smoke detector, if it is mounted on the ceiling, too close to a wall, the smoke misses it! Nah, Forget that!
To the best of my memory, whenever I ran into a soffit that had the vent cups/rings, that I was planning to cover with vinyl, for the most part they were open to the house.
Some were to one side or the other but I think that they really did not think about it. For the most part, they are installed 3/4" above the bottom of the fascia.
I think the direction of the opening is purely astectic. As stated earlier, the attic is going to draw what it needs as long as there is enough opening to alow it to.
I would think it would be more important to make sure you install one vent in between every joist and have a gable vent large enough or better still ridge vent to handle the flow of air. And make sure the attic insulation is not blocking the soffit vents.