JROTC is viewed as an extracurricular. Many people will tell you how it will help you, especially if your unit is a honor unit. Honestly, though, don't do extracurriculars you don't like. The vast majority of USMA cadets never did it.
If your JROTC unit is an honor unit, as Scout mentioned, I would recommend staying in it. I received my nomination to West Point through my school's JROTC unit because it is an honor unit.
Just as a note, any JROTC unit (Army, Navy, Marine Corps or Air Force) that is a current "Honor Unit" can have their Senior Instructor nominate them to any Service Academy.
It would probably help.
But I wasn't in JROTC (in fact my school was way too small to even have one) and I have an LOA and a nom at present. So you don't need to be in it, but have other things to make up for it (e.g. I ran with the ambulance as an emergency medical responder)