No. According to a report issued by the Water Quality Research Council, water softeners do not have any detrimental effect on septic systems and may actually enhance their performance in certain situations by encouraging the growth of additional bacteria.
In fact, the flow from the softener is typically less than the waste water discharged from an automatic washing machine. The studies credited the high levels of calcium and magnesium present (in the flow that results when the softener cleans itself) with improving soil percolation in many instances. These studies were conducted by scientists at the University of Wisconsin – Madison and the National Sanitation Foundation.