I've just swapped out the tensioner pulley for a new ACDelco item (part number 38006).
The old pulley has a lips on each side - the new one doesn't, and the belt's sitting quite near one edge.
I'm not entirely happy, but the new pulley's as far back as it'll sit.

I've attached a pic - we're looking at the AC Delete pulley in the bottom of the image, the new pulley in the middle.

Should I be worried?

 

I recently installed a idler pulley, it was like that. I used a press to move the bearing alittle so the belt would not be too close to the edge.