So I think the idea behind rails and placement as far as sliding goes is to place them so the board sits flat on the sliding surface, or if you have just your deck on the ground, the board will be flat.
I feel like I'm stumbling over my words, but by 'flat' i mean that when the deck alone is on the ground, without rails the deck would be able to rock side to side due to the concave (convex if you're talking about the bottom of the board). I've placed rails as far apart as possible while they still prevent the deck from rocking back and forth.
But in all honesty, whatever you want to do works, and I actually only rock one rail on my toeside edge. Two rails are just too slick for me, but I like the rail for bonelesses, and sometimes I'll ride my board backwards so I can grab onto the rail when doing early grabs with a melon grab or beanplants.
QUESTION - What kind of screws do any of you rail-users use? The stock screws I've had don't work especially well for me, especially the Powell screws (or maybe it's the design of the rail, or a combination of both). The screws will rip out or strip and the rail will come loose or fall off, and make a lot more rattling noise while riding.