I've got quite a perplexing issue, all my other pulleys seem to be 'fine' on alignment of the serpentine belt, Except for my idler pulley which I've replaced because of the bearings going out in the old one and causing a nasty squeak too.

The pulley seems to be 'eating' the outside of the belt, pointed towards the fan.
Not the top, not the bottom groove, but that thin little side.
It's like it's missing a spacer or washer to make it align up.

Would it hurt to put in a washer there so that it bumps it out enough that it stops dragging on the very edge of the belt and eating it?

 

Ya sounds like you need a bit of a space there. I would go ahead and put a washer in. The worst that will happen is that you'll space to far, but just keep an eye on it for awhile after to make sure nothing slips out of alignment.