Making a jig is the absolute best way to insure artwork is centered on a particular item. The outline of the material - in this case the coaster - and the artwork are in the same file. The artwork is centered in the outline of the material. Open your jig file in the Glowforge interface and set the artwork to ignore and the outline of the material to cut. Cut out a space for your coaster out of cardboard or draftboard. Without moving anything, remove the cut out material and place your coaster in the cutout. Then ignore the cut line and enable the artwork to engrave.
Here is a video explaining the process: Video Tutorial - How to make a jig