Having control of tiles based upon what units were last to forcibly occupy them sounds good, when you put it that way. My only concern would be that in a phoney war, if an enemy scout entered your territory and sat on one tile, before leaving, you wouldn't have control of that tile, which would play havoc for your economy until you re-occupied it, although realistically, whoever lived on the tile (if you are thinking of a tile and improvement as, say, a rurally occupied area, or a town) would still be producing for you, after the scout unit had left.