first, know the map, know where usually remote bombs are, since those will instandly destroy you and your uber. So expierence is vital for the medic.
Second a ubersaw it very helpful and use it a lot against spies to uber up since they are terrible in melee, just don't let your guard down.
Third personally I do not like the switching, makes people confused, If I am ubered, then not, then ubered....I won't put my head out there in front of 4 sgs.
Fourth, the top 4 to uber
pyro, heavy, soldier, demo
is really a bad one....
I would say, demo (since there are always sentry's)
pyro to burn the people and push them back if there are not much sgs.
Heavy since he can do a lot of damage as well in front of a lot of people
soldier (only when it is with krits, with regular uber he is pretty useless)
my vieuw on this.