I would definitely not suggest going heavy when doing them behind the neck. If you do, do not bring your arms past parallel. Depending on how long your arms are, this motion might only be to the top of your head (in my case). I learned this the hard way when doing a set of 10 w/ 185. On my 10th one, I wasn't stuggling, but the weight leaned back a little putting my shoulders in an awkward position. I felt a pop in my shoulder, and afterwards thought that it almost came out of socket. I was going down past parallel to the bottom of my ear lobes when doing this. Now when I do shoulders, I use dumbells mostly, because they don't restrict your motion like the bar does.