SJ, Welcome to the board. As the others have said, it depends on you and what you want. Few guys start out thinking they want a longer mustache, but many that let their beard grow decide to also let their mutache grow. So your options now are to trim or not trim. How you choose to style an untrimmed mustache is a different question, but I can say you will have options for a longer mustache and can change the style on a daily basis if you wish. If you have a short trimmed mustache, you have only one style option. Many of us here with long mustaches do not style it into a handlebar everyday. So I suggest if you are unsure, do not trim and start training it to the sides to keep it out of your mouth. You can always trim back later if that is what you decide.

My Beard Journey:  http://jefffsbeardboard.yuku.com/topic/25137/Finally-I-have-a-Beard
Beard and Bald Thread:  http://jefffsbeardboard.yuku.com/topic/27831/Beard-bald-head-shaved-head-or-buzzcut
Patchy Beard Success Stories:  http://jefffsbeardboard.yuku.com/topic/33742/Patchy-Beard-success-stories-Before-and-After-photos#.ViujbH7nuM8
Taming the Wild Mustache:  http://jefffsbeardboard.yuku.com/topic/30822/Taming-the-Wild-Stache#.Vj_Mh7erTIU