I don't necessarily think the dealer is right in suggesting it'll 'move over' or sort itself all on its own accord
if it's really miss-aligned excessively; but really, unless the belt is
being forced HARD up against the flange &/or riding up onto it/being over-heated/burnt by running waaayy too tightly against the flange, then there's
probably not all that much to worry about! :lecturef_smilie:
Your belt doesn't really care where it is on the rear sprocket, just so long as the rear wheel is aligned to aim you fairly straight down the road and the belt runs
ON the sprocket teeth and it isn't being damaged by running part way off the teeth or too hard up against or even on the flange; similarly, the belt really doesn't want to run too hard against either side of the front sprocket or up on either flange up that end either; but the 1-3 mm thing is simply to give you an idea of how well the rear wheel is aligned and some target to aim for when setting the belt properly!
However, the belt
will move around on the rear sprocket somewhat as you ride forwards/turn, etc; and
it'll move around even more as you reverse; so exactly where it is at any given moment is a bit of a moot point and someone ensuring it's aligned perfectly at any given 'non-riding' moment is pretty much bit of a waste of time & effort too, cos
as soon as you start moving/riding again the belt'll be wandering around on the sprocket, at least a little anyway... :banghead:
So don't stress too much just yet - when you can safely do so, ride it a bit, then check the belt's position. Ride it some more, then check the belt's position again. It'll likely be in a little different position each time, so do that ride & check thing to make sure that the belt's still somewhere on the sprocket teeth & not part way off or riding up on the flange, then continue on or adjust as necessary. Do that every now & then for a while (only once you've worked out how to ensure it's a safe/good exercise for your hip tho!

) until you are either convinced that your belt truly
does need to be re-aligned, or you are comfortable that it's just moving around a bit as just about everyone else who's actually taken the time to look/watch their belt like this has discovered, cos these belts really do wander!

And if it's wandering a little or even a lottle, but not running off the sprocket, causing any belt burning, or scoring the belt/teeth, then you don't really need to worry about it. :thumbup:
And if that's the case, you just hafta learn to......
Ride More, Worry Less! 
hyea: