if the cam is right by a doorway or other choke point you wish to monitor, lower is better or you'll just see the tops of people's heads.
if it's meant to cover an area away from the wall, then higher is fine...
nevermind. Yes, for LPR at some distance, the difference in angle between 6 and even 20ft is no problem
(mine are @20ft height and capture plates at about 80 & 100ft, at almost 40 degree angle off the street also)