Backflow preventers are a good idea only if you are below the next upstream manhole. In fact, if your fixture heights are above the level of the next manhole the Residential Code prohibits installing backflow preventers because of the possibility at they can cause sewer gas to back up and potentially explode. Section P3008 of the 2012 IRC, but has been in the Rsidential and Plumbing Codes for years.
http://publicecodes.cyberregs.com/ic..._30_sec008.htm
Remember though, that backflow preventers are devices that require maintenance in order to operate properly. They need to be served by a plumber every few years or they can easily cause sewer backups by becoming obstructed or slipping into the closed position. These valves are not simply one way check valves as another poster indicated, they are a little more complex and have moving parts that need to be exercised and lubricated and plastic parts that degrade overtime.
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