I play with two teams for daily league and cups. The focus on the league is when I'm falling out of the top 4 running, otherwise it's the cups.
Once I pick the two teams for the day, taking injuries into account, I then focus on attacking the opp weakness or defending against their strengths. That is the only time I would reconsider my formation, and it has to improve my scoring and not concede at the back. I do look at the opp past few games to see his formation, scorers, weak players,etc, but otherwise if I'm confident with my players, then let him change formation.