http://www.msnbc.msn.com/id/15939377/ns/health-health_care/t/ease-back-pain-dont-sit-straight/#.TpUf3fIUGf4 says that a 135 degree sitting position is optimal.
When strain is placed on the spine, the spinal disks start to move and misalign. At a 90-degree sitting position, this movement was most prominent. The disks were least moved when subjects were sitting back at a 135-degree sitting position.
"We have to do something that is similar to the lying position," Bashir told LiveScience. Lying down in a relaxed position with your knees slightly bent is the best position that a person can be in, because it doesn't cause any stress on the ligaments, the thigh muscles as well as on the back.
Sitting on a chair that provides proper support, such as a slightly tilted back car seat, can mimic the relaxed supine position. Slouching caused a reduction in the spinal height which means that there was high rate of wear and tear in the lowest two spinal levels.
But it's a single 2006 research study. I tried leaning back and it seemed to trigger lower back pain. And also - it seems that everyone recommends lumbar rolls, but lumbar rolls would make it more difficult to lean back, unless you have a chair that allows you to recline
Reclining backward at a 135 degree seat angle may be "the best biomechanical sitting position" for the back as claimed by the research, but it isn't for the neck. A person sitting at a 135 degree seat angle must counter-recline their head and neck forward at a 45 degree angle to look straight ahead as one is required to do when working in a seated position. Dr. Bashir said "Sitting in a sound anatomic position is essential, since the strain put on the spine and its associated ligaments over time can lead to pain, deformity and chronic illness." I think pain, deformity and chronic illness is exactly what you'll get in your upper back and neck if you sit reclining backward at a 135 degree seat angle.
I've noticed that my computer monitor is placed above my head (difficult), then if I'm reclining at 135 degrees, my neck is going to be at an angle less than 90 degrees (which is bad for the neck)