No need to be nervous what you post, just use common sense. A good rule of thumb is don't post anything you might have a hard time justifying later in court.
Example - You post words to the effect - "Someone comes in my house uninvited he is leaving with a bullet in the head, no surrender at that point". Now, a couple years down the road you are involved in a home invasion shooting. Its not the perps fault - its societies. A good prosecutor will find your ramblings on the net.... "Ladies and gentlemen of the jury, the accused murdered the victim in cold blood with premeditation, let me read you a transcript, in his own words where he states....."
The Internet has been "watched" for as long as it has existed. As far as using a Govt. computer, that's really giving up any possible right to privacy, not to mention a code of conduct violation in most organizations if it is being used for personal use on Govt. time.
Larry