The 50mm 1.4 has a very shallow DOF, so depend on what angle you take, and whether you are particular or not, you might not notice the back or front focusing problem. As the lens is within warranty, Sigma will calibrate for you with your camera without charge. I am sending my next week. Heard from technician just 2 hours only.
I have played with the Nikkor AFS 50mm too, and notice that focusing are off, and can conclude that it depends on how wide you want to open. If you shoot at say 1.8 or 2.0 you will not have such problem. This is my 5 cents worth of advice.