Fixed Units - How Should They Be Managed?

[Elxaime]

Most Panzer Campaign game scenarios have fixed units.  They are placed in this state for a variety of reasons, and usually (but not always) have timed releases.

I have often wondered what the most historical way is to treat such fixed units.  Some designers provide house rules specifying they should/shouldn't be used in certain ways.  But usually, the player is left to do as they please.  Hence, fixed units can dig in, lay mines, build bunkers, blow bridges, and, if indirect fire capable, bombard spotted enemy units. 

However - again thinking of how best to approach this historically - I have often thought the better approach was to leave fixed units be unless/until they are released, with the sole exception of indirect fire units, which can bombard.  

How do other players approach this?