Enclosure and pcb ground
The first thing you should worry about is proper grounding on the board.  Think about the high frequency current loops carefully.  Keep them small and off the master ground plane.  A shield should be considered the last resort for EMI problems.  It's better to not make a mess in the first place than to try to clean it up afterwards.

If you do have to use a shield, it should be tied to the master ground in one place only.  Also keep in mind that every opening in the shield becomes a slot antenna.