Fairfax County PD spells this out in their General Orders, stating:
"Because conditions vary with each roadway and the primary purpose of radar/LIDAR is to cite the more flagrant violator, no motorist should, under normal circumstances, be cited for violations under 10 mph beyond the posted speed limit. Tolerance levels, over and under the minimum, may only be set by the station commander based on circumstances of a specific location."
Reference:
http://www.fairfaxcounty.gov/police/inside-fcpd/pdf/generalordersvolumeiiprint.pdf
If for some reason you are pulled over for less than 10 mph over the limit, this document won't help a bit in court. And it only applies to Fairfax County, not the many other police forces in the area.