I always go for the precision (tight fit) which is really what we should aim for with joints, otherwise why go to the expense of a Domino?
I have various sized slightly sanded Domino’s to test if in doubt, if there is an alignment issue, the mortise can then be opened up a bit but, my preference is snug as a first option.
I would say, practice a little with some sanded Domino’s, and in no time you will become more comfortable with a firm joint.