change formation or use only one?

[sincity]

I think the answer depends on what time a manager can afford to spend on the game and how good is the team. If you have a stronger than your league opponents' starting eleven, than you should not bother changing formations. You could just play some versatile formation like 4-4-2 or 3-4-3. But if your team is of less quality or of the same equality than the ones you come up against, than using the right formation every game will be very helpful. Or at least that's what I think.