this is personal opinion, for what it's worth. If you want mouse buttons for mousing, hook them up, if not, then you have the hassle of having three buttons connecting through 1 interface and all your other buttons connecting through another. For the sake of simplicity, it will be less work to have all your buttons on one interface, which also means if you decide to trade in your spinner for a trackball or some such in the future you don't have to rewire those three buttons.
kamakazi