Presumably there are pre-drilled holes in the board??
If there are, and the enclosure is metal, I would always go for an insulating plastic spacer. The simple reason being that with an insulating spacer, you know , absolutely, that there is no electrical connection between the case and and any PCB tracking close to the fixing hole. This means that you do not need to check pillar-to track -spacing, and you can be sure of the true earthing (grounding) of the metal enclosure (or lack of it).
One word of warning... some plastic spacers have brass bushes at each end, or studding moulded in in such a way that there is still metal on the spacer of a larger diameter than the fixing holes on the PCB These can short out tracks on the PCB if the guy who laid out the board put the tracks too close to the hole.