it depends on why they're going after your medic/how many people they send. if it's a single bomber trying to force during a stalemate, you should always be able to deal with him (especially if you play close enough to deny entrances). if they're chasing you with them having uber and you not, you should recognize that ahead of time and be far enough back. if you just killed their medic and they're trying to equalize the advantage, you should have a lot of dudes on your team come play heals and deny all bombers.
etc.