you need to run the wood through a Planer and a Jointer to ensure they are as true as possible
also make sure you are using flat MDO / Plywood, or as flat as you can get
Even MDO has warping in it
To get truest flat surface possible, we made the wood frame and then took a solid of sheet of aluminum, .125 lay that on top of the wood and we use PVC 12mm for the cutting surface