From: Bedfordshire UK
You could say that, as with other things in this game, most garrisons are abstracted. The unit counters on the map represent those field divisions and korps that are part of the mobilised fighting forces, but there will also be security, supply, training and admin troops behind the front. If you choose not to put fighting units in cities, then there is a risk that they could be captured if near the front.
I find myself, as Axis, putting units in all ports and cities near coastal areas and in all cities near the front line, otherwise I don't try to have units in cities, except that I have units deploying into cities, or using cities during the frozen turns after upgrade to korps level.
"We have to go from where we are, not from where we would like to be" - me