I learned something about that when we built our house. We used spray foam insulation, and when the A/C guys were installing everything, they asked us how we wanted the vents pointed. I asked him what he recommended, and he said that normally, he recommends pointing them towards the walls because the walls get heated by the sun, and if they are pointed towards the room, it will take longer to cool because of the heat absorbed by the wall. He said that because we were using the spray foam insulation, it made less of a difference because the interior walls wouldn't be as warm.
One thing I'd add about the spray foam/traditional insulation debate. While it isn't relevant to the OP because he was only looking to do the attic....Something that never gets mentioned in the debate is critters. When we had spray foam insulation, we had almost no non-flying critters in the house. We saw no spiders, scorpions, etc. Since the foam seals everything up, there are no pathways for the bugs to get in.