I would agree with all Robin has said above. I would,however, caution that you need to check what your processing is doing what you expect and not adding any anomalies.
While I have not done this yet I am considering taking the mean of my bias frames and subtracting it, rather than the bias frame, from the other frames (targets, flats, darks). The logic is that for CCD (but not CMOS) the bias or offset is not a function of the pixels but of the output electronics. It should, at least in the short term be constant, which is estimated by reading out an unexposed frame. Some amateur cameras and professional ones have an over-scan region for this purpose as it avoids any residual dark current light leaks etc.
If this is a valid approach, and I think it is, it avoids adding in noise during the bias removal. Thoughts welcome.
Regards Andrew