I have a 2008 S40 with about 500 miles on it.  Right out of the showroom, I noticed that the drive belt was running on the rear sprocket/pulley to the outside, touching the lip.  The sprocket/pulley is wide enough to have a little clear space on both sides of the belt before it hits the lips.  My belt is not showing any edge wear.  How important is it to adjust the belt to run perfectly in the center of the rear sprocket/pulley?  Manual says use the marks on the axle adjustment and match both sides.  Sounds a little crude to me.  I noticed after only 250 miles the belt was loose enough to not make the first ring on the adjuster checking tool  Took up each side equal number of turns.  Of course, belt still runs a little to the outside.

Let me put a different spin on what Yonuh said.
Someone on fleabay sells those gages for about $10 each + shipping.
hint, hint, hint.
The rubber dampers give the sprocket a soft mount to and give a little with each pulse of the power stroke.  For this reason the belt will run left and right while riding depending on how much power you are applying.  There's a thread that will show how to align your wheel, but don't expect to look back and see the belt dead center all the time.

Bear,

Just as they say you should not trust the belt tension tool in the toolkit, you also should not fully trust the index marks on the swingarm.

You can try this:


Cheapo Rear Wheel Alignment Tool


Works on all of my belt and chain-driven bikes.

IHTH!

I went & saw what Digger had posted & I replied to Ralfguy, who I had failed to respond to.

I sure cant fault Serow's idea, put the tire in line & let the belt do what its gotta do. I have been working from the assumption that the wheels would be aligned IF the pulleys were & maybe thats just not so, IDK>

Anyway, here is my post from Technical, & I like that tool Digger has. It looks pretty handy for lining the thing up.


Dang Ralf, I never saw your reply, Im sorry. Let me see if I can make that more clear.

In the oil field, big pumps are belt driven, the belts, if cut & laid out straight, would be around 40 feet long, so the pulleys are very big & the distance from power point to the pump is quite a ways. In order to line the pump up with the drilling rig, we would use a string, held tight across the edge of the power point on the rig, that showed us where we had to get the edge of the pump's pulley, to line the belts up. They used 6 belts, ganged on these big pulleys. I used to toss old gloves in just to watch them get torn to shreds.
So, a string, pulled tight, makes a straight line. By using the belt to pinch one end & roll the bike so the belt takes the string to the farthest point forward possible, then pulling the string back & across the rear pulley, all the way across it, not just back to it, then the edge of the rear pulley can be brought to alignment with the edge of the front.

Once the rear is adjusted to satisfy, marking the rear adjusters might be a great idea, because as Digger has pointed out, they arent marked exactly evenly.