I'm pretty sure that this goes out to customers with a store that plans on closing at their regular time (probably 9 p.m.) and then reopening for the midnight release. They want you to check-in and probably pay off the game, that way they do not get rushed at midnight with people trying to pay.
In the past, the ones I have been to, everyone pays before and gets in line with their receipt. At midnight, they start filing people in. Someone checks the receipt at the door, you get sent in, and someone checks your version at the counter (ps3/360) and they give you your game and you walk out. It greatly speeds up the process rather than people trying to pay and get the game at midnight.
So yes, go in before 9 p.m. and they will tell you when they reopen and when you should come back and get in line.
No you don't have to but it helps speed up the lines faster by getting the final receipt. You can still show up at 12 with no problems and not every Gamestop does the numbers either. Mine never does, everyone lines up and waits. I always shows up right at 12 for big midnight releases like this, it only takes about 15mins to get through the line.