Any additional axis you put on your machine will cause a loss of rigidity. Nothing is infinitely stiff, so anything you add will allow a bit more movement for a given force on the cutting tool. The question is really how much movement your particular mechanism will experience with the forces you will put on it and whether or not that movement is acceptable. I'd imagine the best people to ask about this would be the Doughty drive people. Tell them what you want to machine and see if they recommend their products for it.