I bought a Tractor Supply PHD specifically to plant trees. Using both a 6" and 9" auger's, I've sucessfully planted 60+ trees, a combination of hardwoods and pines. Some have grown upwards of 10 feet now......
I typically run the auger all the way down, which is way deeper than what the bare-root trees I plant need. Then I back-fill with some really good soil that contains a lot of organtic matter. I always put a fertilizer tablet in the bottom of the hole and water everything really well.
Also, if you have clay soil like I do, the auger will tend to create "smooth" sidewalls in the hole. Take a shovel or digging bar and score the sides of the hole. I've been told that those smoothed-up sides will tend to make it difficult for your trees roots to penetrate.... sorta like being pot-bound.