Cutting down your HSTs is done to be sure you have a full seam allowance and accurate 6.5 inch squares when complete. It also gives you a bit of wiggle room if your line of sewing isn't completely straight...say the last 1/2 inch of your 7x7 seam is a little wonky, you can avoid redoing the block or seam ripping by cutting the wonky part off.
There are many ways to do HSTs, if you aren't happy with this one, use your method of choice. As long as you've got the 6.5 inch completed square, how you go about getting there is personal preference. I hope that helps.