There are some units, archers I think and crossbowmen that have benefits for shooting if they are not in combat.
Remember a unit must have line of sight, and it specifically says models in their own unit do not block line of sight. So you could argue if you are in combat, then models in front block line of sight. I'd certainly say so if your shooting unit is smaller than the engaged unit, but probably not if it was the other way around. Taller models should be able to see over. And of course you would have line of sight of the unit directly in front of you, so if I were to houserule, I'd just say units in combat, can only shoot at units they are in combat with