If you're going to mount lights by just drilling into the bumper there is a black plastic under-bumper part that backs up the grey bumper cover in some places and not in others. Your light mounting will be more rigid if it goes through both pieces of plastic than if you just drill a random hole in the grey bumper cover. Take a look from underneath and you should be able to see the stronger overlap places.
I marked my bumper cover when I had it apart. From the center of the Jeep to the sides the strongest points are 6", 11", and 15". Also measure 3" out from the vertical lip at the back of the top of the bumper cover. The lip is hidden under the grill.
Check that your light housing doesn't have interferance issues with the grill due to depth. Also look underneath to check access to light fasteners. Accessibility varies between locations and there is a little wiggle room on the hole locations.
I haven't drilled these holes in my jeep so I don't know how well it works or lasts but it's gotta be better than betting on the strength of the grey bumper cover. My plan is a removable lightbar in the locations for a tow-behind bracket I installed.