Our company policy is that if you give them a "heads up", they pull you off immediately. Once the words "sick" or "fatigue" get thrown out, game over.

If you go on tape as being sick, even if you think you're good for one more leg and something happens you will probably be violated for intentionally flying sick. Intentional violation = no ASAP/ASRS.

If you're going to gut it out, just call in when you get there. A "heads up" helps the company but at the expense of risking your certificate.