It's probably more important that the plane of the rear wheel be in the same plane as the front wheel. Or at least as close as can be. Less important that it line up with the frame. However if you've been riding it and there aren't any odd noticeable quirks............ who's to say? Maybe just faster tire wear than others.
Without knowing how the axle is set up on that bike, I'd suspect like a few others that a spacer was put on the wrong side or something.