The center panel on a raised panel door is designed to expand and contract with seasonal moisture changes. Sometimes a panel that was stained in a humid environment will show an unfinished line when moved to a less-humid environment. This unfinished line can appear around the edge of the center panel and detract from the overlook of the door.
To prevent this problem you can stain/finish the edges of the panel before assembling. The door assembly can then be glued, sanded, and finished without any worries.