The extra stitch they want you to create at the point of the v neck is so that your corner will turn properly. It needs that extra boost to “turn” with. Just be sure when you are all done will all the picking up and inc 1…and before you commence with the 2x2 rib…that your stitch count is a multiple of 4 +2…which means that you will be able to work “K2-P2” to the end of the row, and end with an extra K2, right side facing.
Don’t pick up too many or too few stitches. If you pick up too many…your band will ripple. If it does, you have to rip it all down and re-pick-up less stitches. If you pick up too few, you neckband will respond by curving or curling sideways in a fetal position…also aggravating.
This is the curving that I mean in the photo below, that I pulled off the internet. She either picked up too few stitches, or she cast off the last row too tightly. I use a size 9 needle to cast off if I’ve knit with a US7…two sizes bigger.

Here is another one. She how she has to yank on the buttonbands to pull it down? Even then it is not working out.
This is definitely too few stitches picked up. If you use wool, you can sometimes tug the ribbing out during blocking. Or, re-bind-off, using larger needles.

This one, below, is perfect…and I’m sure she blocks her work, too! Very polished looking!

The appearance of buttonbands ribbing is a pet peeve of mine. If it’s done badly, it’s so ‘hanging out there in the wind’ so to speak. I’ll tear down a rippley or curvy (whatever the case may be) buttonband in a heartbeat… and RE-DO it til it’s right. I have good days and bad days!
Just a little tip: I always divide the sweater’s raw edge into evenly spaced sections, marking them with safety pins, before starting to pick up the stitches. Then I make sure that the right amount of stitches are picked up within each section. I try to pick up the total # prescribed by the designer…but that is just a starting point or guideline for me. Sometimes my sweaters are longer than the pattern model…so my raw edges are lengthier, requiring more picked up stitches.
Well, I am talking way too much. I hope something I have said will help you! Feel free to PM with any particular questions.