I've done this a bunch of times and have gotten good results. As Terri points out above, you can lay them on their side while you harden them off, which will cause the plant to develop a nice bend in them making it easier to bury their stem just a few inches deep along the surface. In my opinion, the advantage here is gained by increasing the amount of surface feeder roots that the plant creates. When I've simply planted them deeper, I haven't noticed an overall deeper root system when I pull them out in the fall. I think that deeper stem simply dies back.