With only 2 mm of material left on the board edge, I would go for the routed corners and allow a bit less than a mm (25 to 35 mils) of clearance around the posts. (If the board edge clearance was greater - say, 3 mm or more - I'd probably favor the holes.)
In my experience with molded plastic cases, the combination of size tolerance plus locational tolerances makes the case assembly pillars shift around with respect to the PCB mounting bosses. If you use the case assembly pillars to position the board, then you need some oversize holes for the mounting screws; if you use the mounting screws to define the board's position (my preference), you need a little extra elbow room around the case pillars.
Also, during assembly, it's easier to drop a board in place when it has corner cutouts rather the getting all the pillars aligned to slide through their respective holes. If you're using the pillars to set the board's position inside the case, you WANT the closer fit around the pillars but in most situations it's easier on the assemblers to have more clearance around the pillars.