Actually for Bayern from the 5th match they are also safe so they are not too eager to get 3 points in the previous match because after all they need to save energy for matches that are not too important.
You are right because with the points Bayern Munich have right now there focus is not much on there match against Manchester United because they will like to reserve most of there key players so that they will have enough energy to perform well on there next round, so that will make them use there younger players against Manchester United.
So however if Manchester United utilizes the opportunity and win Bayern Munich they could stand a bit chance of hopping for the second position if Galatasaray and Copenhagen plays draw.
Actually, what you said is correct: Bayern will not focus on winning or losing in this match. But I believe they will still bring out the official lineup for the game. Their elimination of MU is also something that needs to be done because MU is still a heavy opponent in this arena. They will still play their best to bring home victory, helping the whole team have the spirit of winning and being undefeated this season.
MU's victory over Bayern was difficult because, in the first match, they were defeated by Bayern. There will be no chance for them. Hopefully, MU will try to protect the goal and not concede too many goals. This will shock MU fans.