The answer to this question varies depending on the nature and outcome of the back surgery.
Regardless of the the surgical procedure that has been done, we will generally wait at least 6-8 weeks after a surgery before doing any type of manipulation near the surgical site.
If a spinal fusion has been performed and the recovery has gone well, those areas are usually safe to work on. If you think about it, once two vertebrae are fused, they effective become one big bone, and there's no chiropractic adjustment that's going to re-separate them.
The interesting thing about spine fusions is that the the joint above and the joint below are now taking on the strain of the now-fused joint. It becomes even more important to have proper spinal alignment to prevent future problems or premature wearing down of the joints.
One feature we have in our office is instrument adjusting, which requires no twisting or popping of the joints. While most people tend to prefer the "good old fashioned" style of chiropratic adjusting, we do have tools available that work especially well post-surgery.