Plains should be able to shoot at units in cover but they should lose points on their dicerolls cause they cannot see to clear where their target is they just know it is somewhere around in that hexe.
When plains attack units which are not in cover then they should have no penalties to their dicerolls.

I would go for the following deductions:
FIGHTERS:
-1 for attacks in the hills
-2 for attacks in the woods or cities

Divebombers:
-1 for attacks in the woods
-2 for attacks in the cities

Bombers(not sure there are any bombers yet):
-1 for attacks in the cities