Never C-betting with air.

[PantsOnFire]

There is no easy answer to this since a lot of your play will depend on how your opponent plays. And even if you are heads up against an opponent who has no observational skills at all, the other 7 players may be intently watching you.

Only c-betting with something against a good opponent is going to be unprofitable very quickly.

And keep this in mind. Even against a calling station that will call every single c-bet you make, there are times, especially if you are raising narrow, that you will both have air but you will have better air.

In holdem, "never" is usually always a bad path to go down.