Pretty good run.
The ET clock does not start counting till you move no matter if you sit there for a minute your 15.32 will still be a 15.32. Your reaction time clock starts as soon as it turns green...which means if you are actually racing yes you better cut a good light.
For racing lets say you dial your car in at 15.32 and have that written on your windshield, you have to run as close to it as possible without going any faster than that. For arguments sake lets say both you and the person you are racing against both run exactly on your "dial ins" (whether his dial in is 16.5 or whatever) the race will be decided by who had the better reaction time.
In bracket racing it is good to race a slower car because the computer will turn their lights on accordingly so they get a head start making it so you both should cross the finish line at the same moment...which means they get a head start so you will be able to see if they redlight. If they do you can take off safely not trying to cut a good light as you have already won the race and will move on to the next round.