As far as I understand it, Sigma corrects the problem under the lens warranty. However, if the lens is brand new, I would just ship it right away to the shop I bought it from for an exchange. Now, if you have a camera that back/from focuses with more than one lens, then there is the possibility that the camera is causing the problem. In this case you may want to send both the camera and the lenses for calibration. I would not send the camera and one lens for calibration, because it's possible that while the lens I send in is now calibrated, the other lenses won't.
That's where the lens micro adjustment in some Canon cameras becomes useful, since you can match all the lenses to the body. But again, it's well known that some Sigma lenses may front/back focus, and that's why it's best to test the lens carefully following the instructions, and then send the photos and the lens for calibration. More than likely you won't have to send it in more than once if you test the lens in accordance to the instructions. As with any tests, one would have to use a rock-solid tripod, set the lens to the right aperture, focus point, distance from the center of the lens to the center of the chart, correct angle between lens and chart, lens on AF for a couple of photos, and then a couple of photos with lens focused manually.
This chart was designed for Nikon lenses, I believe, but it can be used for any lens:
http://www.dphotojournal.com/download-free-focus-test-chart/