Sitting straight upright in your chair is bad for your back.
You should instead slouch at an angle of 135 degrees.

Scottish and Canadian researchers used a new form of magnetic resonance imaging (MRI) to show that
Sitting straight places an unnecessary strain on your back.
Levent Caglar from the charity BackCare: "In general, opening up the angle between the trunk and the thighs in a seated posture is a good idea and it will improve the shape of the spine, making it more like the natural S-shape in a standing posture.

"As to what is the best angle between thigh and torso when seated, reclining at 135 degrees can make sitting more difficult as there is a tendency to slide off the seat: 120 degrees or less may be better."

Dr Waseem Bashir of the Department of Radiology and Diagnostic Imaging at the University of Alberta Hospital, Canada, who led the study: "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."
Rishi Loatey of the British Chiropractic Association: "One in three people suffer from lower back pain and to sit for long periods of time certainly contributes to this, as our bodies are not designed to be so sedentary."