I find topics like these interesting, because playing TF2, for me, is less pre-arranged tactics and more situational awareness.
Pre-arranged tactics only work if the whole team is in on the tactics. A football team can call "Red 22!" and everyone knows that the plan made in the huddle is now modified to have a different wide reciever, or whatever that code means to them.
But generally, in TF2, short of shouting "Scout rush!" or "All Engies!" there isn't such a high level of planning involved.
When I'm playing medic, what I do in the next second is dictated by the situation I'm in. Becuse there's a few things to consider:
1) Healing somebody gives you uber.
2) Overhealing somebody gives you uber.
3) Healing somebody taking damage gives you more uber than overhealing.
4) Everyone hates being on fire.
5) Dying ruins your uber meter.
6) Some people suck as medic buddies.
The other key is Voice Chat. If everyone is on voice chat, medics can be better medics.
So, if I has been building uber, and am being a medic buddy with a heavy, and a pyro alights both of us, first off, I shouldn't have been so close as to make it that easy. The medigun has long range.
If I'm lit up anyway, I think... Is there a medic kit nearby? How long would it take me to get to it? How much uber do I have? How hot is the action (how likely is it I can even get to the medpack?)? Is there another Medic on my team? Is this Heavy so awesome that he'll do more by being put out than I can accomplish by staying alive? Is the heavy listening to me scream for health?
All those thoughts integrate in my head. If I have high uber, and the heavy is unresponsive to my pleas, and health is nearby, and I have a chance to get it, I ditch the sucker.
If the Heavy is participating in spawn control, and I don't have a lot of uber, and I can't get to health soon enough, I stick it out and spam the medic key. I'll even say in voice chat "I need medical backup at RED spawn!"
If the Heavy is decimating a scout rush, and I'm almost dead, and I have a lot of uber, and we're both on fire, and I don't have chance of getting to a medical kit, I stick to my guns, shout for help, and die.
In all cases, if I'm following a player who treats me like nothing more than a mobile health kit, who drags me into dangerous situations, who doesn't pay attention when I need health ("Medic Buddy" works both ways... I take care of you and you take care of me), I'll buff 'em during setup or when I'm charging Uber, I'll take the edge off their damage, I'll put 'em out, but they're at the bottom of my triage list and I never follow them, and rarely give them Uber (if they're the only heavy in a team of snipers, I swallow my pride, prepare for death, and give em uber.)
Tactics are great if you can get everyone to follow along. In regular TF2 sessions, however, the best you can ever do is sitational awareness.