BILL– Unfortunately, since “rising trot” is specified, when a rider sits the movement, she is technically committing an “error of test.” The judge may blow the whistle, deduct 2 points, AND make you redo the movement rising. Only that time would be scored. In addition, these would be cumulative errors; so a second time would be a minus 4 (total of minus 6 so far), and the third time would be elimination. Sitting for a few steps (as before a canter depart or in a downward transition) is fine or even in a moment of extremis like shying.
If the judge doesn’t penalize you, consider it a gift, but within the rules she certainly can so I wouldn’t go out on a limb and make it a habit. By the way, the same applies in a more advanced test where the rider is supposed to sit (like the medium trot in Second Level) and chooses to post instead.