I'm not sure how often it works out well to do mirror image by sign reversal. If it matters to you whether the tool is climb milling or not, then it most likely means that you would want to redo the toolpath directions in CAM. But for hole pattern type positioning, it would probably make sense to use mirror image on the machine.
First you get good, then you get fast. Then grouchiness sets in.
(Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)