FO's or On Map Guns?

[Ratzki]

At the battalion and company level, is it more accurate to represent light artillery such as 81mm Mortars and 75mm Guns as FO's or as actual guns and mortar units on the map? I understand the higher echelon artillery being represented by FO's but when these assets were being held at lower levels, how were they most often used? This has been a lingering question of mine for some time, and the game seems to have them willy-nilly shown both ways in the unit purchase section. My gut feeling says that they should be represented as units on the map, but does anyone knowbetter?
I think this would be a minor help to the scenario designers that we have here.