While that may be a part of it, the reason cops do not write tickets for guys going less than 10 mph over the limit is simple...$$$$. The big fines are usually for > 10 or 15 mph over the speed limit. Speeding tickets are about making money, not about public safety.
No cop is going to waste his time pulling someone over for 5 mph over the limit as it is not only potentilally challengeable in court, but it is simply not nearly as profitable as getting the guy going 75 in a 55 on the highway.
And as to accuracy, as they say innoncent until proven guilty except wrt to traffic tickets where Napoleonic law applies in most states. Most times, when people get out of a moving violation by challenging it in court they plead to a lesser, non-moving violation that probably has nearly the same fine but does not put points on the license or affect insurance rates....I.e, the accuracy of the gun is really not going to be a major factor regardless, although it could be in some cases where you can prove that the cop did not handle the instrument properly, etc.