Level is relative, if your spindle/gantry isn’t level to the wasteboard then you’re going to have cuts of various depths. My wasteboard had a .030" dip in the middle, so some shallow carves would leave the middle untouched.
@rpegg’s suggestion is good, and it’s an easy and quick way to see how relatively level it all is. You may want to consider purchasing a large wasteboard cutting bit and leveling it using the spindle. I attached a piece of 1/2" MDF to the top of my wasteboard and used a 3/4" bit to make a nice flat pocket. In case you want to see what I did, I made a video of the process and put it in this forum: X-Carve Maintenance/Troubleshooting Videos - Add Your Own!