It's all situational man.
But the general rule of thumb is you should only leave your medic when he's got a scout around him - or maybe on very special occasions with your demo.
The only other time it's really a good decision is if you *have* to die to keep him alive. Then tell him to run.
Ideally as pocket you'll extend, during which times a scout or somebody takes some heals, and then retract for more heals, or extend, and then never be seen again. The not being seen again will only work if your med has people with him.
With the exception of mids. If your medic is healing you at mid and you're not the best soldier to ever play the game he/she's wasting his/her time.