On our installs we document the locations of our valve boxes. There have been times even with documentation I have trouble finding the valve box I installed that someone else buried after a few years.
just how it goes sometimes
On service calls we don't locate valves unless it's needed. When it's needed I do document the location of the valve.
I personally visit at minimum 2,000 houses out of a year out of our large enough customer base. There is no way I could keep track of all the valve boxes for these houses even with documenting them.
we do offer a much more detailed service call where the valves are indicated in a map of the system