Waste - that sounds like good advice, but is not how my AG pool's instructions were.
Here's what my AG pool instructions said - cut the skimmer hole even with the wall hole EXCEPT along the bottom where you are to give it an extra 1/4 more liner up into the opening (looking at it from the outside, you would see 1/4 inch of liner above the bottom edge of the skimmer). poke through with your knife at each screw hole, hold the skimmer on the outside, fasten your 4 corner bolts by finger, insert the remaining screws, work your way from bottom center and top center outwards alternately doing the corners last.

For the return, cut the hole almost identical leaving a 1/16 to 1/8 half moon sliver visible around the bottom of the return hole and install your return.
On an AG pool, you can't install the return before cutting the hole since it threads THROUGH the wall, and doesn't fasten with screws around the perimeter.

I believe they want you to leave a little extra to accomodate shrinking and stretching - most liners on AG pools will settle some due to the sand compressing over time. I've seen it where the liner pulled out at the bottom of skimmers because no room was left for stretching.