If you're speaking of Fairfax County police, you're generally going to get a bit more latitude than 5 MPH. They actually have this policy in writing. See general order 501.3, section II.H here:
http://www.fairfaxcounty.gov/police/inside-fcpd/pdf/generalordersvolumeiiprint.pdf
The relevant section says:
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.
This isn't a guarantee, of course; if an officer wrote a ticket for 5 MPH over the limit, the general orders wouldn't do you a bit of good in court. And the orders explicitly acknowledge that station commanders can designate some locations as having lower tolerance. But it's useful to understand that under normal circumstances, officers have explicit written instructions not to write citations for violations of less than 10 MPH. The same orders generally disallow setting up speed enforcement at the bottom of a hill, by the way.