True, John, it is always possible to have a varying result when re-cutting. But, that begs the question: Which cut was the miscut? The first one? The second one? Both? I have generally found that re-cutting might make the mortise a bit looser but not objectionably so. Given the oft heard complaints that the dominoes are sometimes too tight I don't think it is much to worry about. In fact, re-cutting is a good way to self-check your technique. If I can make a cut, remove and reset the machine, cut again and still have tight fitting dominoes, then my technique must be good.
You were right on about not changing the width setting. The added benefit of using a new positioning of the mark is that you should know pretty closely just how much you are lengthening the mortise. I don't know how you would do that with a cam adjustment. In fact, there is a scale on the fence that can be used to offset precise amounts from the original center line.