You don't *need* to change formations each time. If your team is better, then the odds are you'll likely be able to win. If you are of equal or worse quality though, it's common sense to adjust your formation slightly in order to take advantage of the other team's weaknesses. Though if you will be playing with a considerably worse team in order to do so, it may be smart to just stick to your ideal setup where your players play in the formation with tactics that suit them.