Actually with the long side down, I'm not sure if that's called slashup or slashdown, but with the long side down, if you are on a slight down hill incline and it is raining, say at a PGR mission, rain can get in. Enough for problems? Nope. But something to consider none the less.
On a bagger,slash up is the only way it looks clean.
I have slash-up on my '07 Ultra and I agree that it follows the form of the saddlebags. Looks better.....
The parts people did say that the direction of the slash (up, down or out the side) can make a difference of what the rider hears. What do they know?
Don't know but I like the looks of my slash-up.