I do this and I feel that it is a good option. I'm still new to the HP game so I'm still working out the kinks in my attachment, etc., but I know that I feel very glad I don't have to worry about my HP hairline showing. Attaching behind my tranplanted hair gives me a little comfort in knowing that at least that at least the front of the HP won't be detectable.
I got transplants a long time ago and recently decided that I'd like more hair in the crown and on top. So I now shave that area and attach behind about a 1.5 inch hairline. It seems to be working well for me. One of these days when I feel like I've got the whole routine down, i'll post a pic.