Should the safe in a retail insurance office be visible or hidden and
why? It seems that a visible time-lock safe shows potential burglars
that it will be difficult to rob the office. A hidden safe may make
the location mysterious. What is the best practice?

I expect that there would only be petty cash in the safe, hardly worth
a burglar's effort.  If you have a street front office, leave it in
sight with a light on it.  A movement alarm system would scare a
burglar off, and maybe keep him away from laptops, etc., that would be
worth more  - and a greater inconvenience and loss of information.