Often the edges of the quilt have stretched a bit while you are working on your top. If you add a border the way you suggested, you are adding a little more length each time you add a border. I once had a customer quilt where she had SEVEN 1-1/2" borders. By the time she finished adding them all, the edges of her quilt measured almost 5 inches more than the center portion!! She did not want to re-do them, so it made it VERY difficult for me to quilt the borders and get them to lay flat.
I have a handout on applying borders on my website that you can print out for reference. It's located on the Tips & Hints page. Once you see how nice and flat your quilt is if you measure borders when applying them, you will be happy to take the few extra minutes it takes to do it.
Andi