yes you need one, first unscrew the reg and look into the frm, does it have an internal thread for the pcv assembly to screw into, if so it is a 2.1 body, if not then you will need the older "golf tee" pcv. the reason for fitting one is to stop the marker discharging gas down the barrel when you degas it, which can sometimes push a ball out of the barrel.
once you have worked out which sort you need (if you need more help just ask) then give planet a call and Stoney will be able to sort out what parts you need and prices.