Thats hard to answer. I'm a hobbyist, bought the Domino and the cutter/tenon kit when it first came to the US (3 years ago??). The assortment is a very good deal financially, even if there's a size you might not use. I work mostly with hardwood in various thicknesses, wound up buying additional 8x40, 8x50, and 10x50. I've used relatively few of the 5mm tenons. I agree with the others above that having them around just in case is a great idea. Once you start using the domino you find lots of unexpected uses for it!
The cutters last a long time, so unless you develop a habit of dominoing thru nails or screws [embarassed], you may never have to buy another one...