Checking the Metric box will change the format of the nc code to metric mode instead of inch mode. It will not change your feed rates or Cutter Compensation or your part size! If you draw in metric, you should use METRIC tools, feed rates in millimeters per minute, etc. The purpose is to insert the code that changes the way the control reads the program from inch to metric.
On most controls, the metric G-code is G21. The inch G-code is G20. Some controls use different G-codes and your post processor should output the code appropriately.