You might have seen the somewhat humorous incident where NFL wide receiver Chad Johnson, formerly known as Chad Ochocinco, celebrated a favorable outcome in court by patting his attorney on the rear. The judge didn't think this was very funny, and decided to withdraw approval of the plea agreement and sent Johnson to jail.
This led many to ask, "Can the judge do that?" The short answer is yes. An agreement that is worked out between the defense attorney and the prosecutor is actually just a proposed plea agreement which is subject to the judge’s approval.
Usually, the decision to dismiss or amend charges is up to the prosecutor, but the sentencing is ultimately up to the judge. Sometimes this works in favor of the defense, since a judge can give a lighter sentence than the prosecutor wants. But, as in the case of Chad Johnson, sometimes a judge won't approve of a deal agreed upon by the defense and the prosecutor.
So, if you're not sure whether or not the judge is going to appreciate your particular style of celebrating or congratulating, it is best to wait until you are outside the courtroom.