Good managers like Mourinho, Ferguson, Wenger ect will still change their tactics to cope with whoever they play against. Tactics used against Liverpool for instance will differ very much from tactics against say, fulham or birmingham. Granted the players on the pitch are the same players but they will have different instructions. Also, most teams they play against have to look how to cope with the likes of Rooney, Adebayour and Ronaldo, so thats a contributing factor.
Rafa Benitez is a class example of somebody who has a great squad, with a lot of tactical naivety. This can be construed also onto FM, as if you go out with an all attacking formation against the likes of Man Utd, you will most likely get caught out. However, if you do the same against Birmingham on FM, you will dominate the game and still get caught out. In my opinion, try to keep similar formations, and vary attacking or defending styles, to keep the AI on its toes. | 
