If any toothed belt is tracking on pulleys that are not in perfect alignment it will walk to one side or another. Typical on Harleys, the belt will walk toward the rear tire when it is rolled backward. Since you don't spend much time in reverse, this isn't a problem. On wide-tire conversions, this is something I aways tell people to be careful of. You're fine. Just keep up that regular maintenance, and keep going forward.
Of course you can also look if you somehow can improve your rear-wheel alignment...
I had my Harley-Davidson up on the lift the other day, and after I was done installing a new primary chain, I was giving it a broad look-over and, doing so, spinning the rear wheel by hand. Then something caught my eye. It looked to me like the drive belt had moved over, so to make sure my eyes weren't fooling me, I spun the wheel forward, then a few revs backward. And I'll be damned, but the belt did track inward toward the rear tire!
What's up with that? My bike has never been apart for anything major that would have messed with the belt. It's still in good shape, and all I've ever really done to the bike is change the tires and do basic maintenance. It this some weird phenomenon, or have I screwed up something? The bike runs and tracks well, so what do you guys think?