I commission in about 3 months. So basically I will have had a little over 3 months to decide whether or not I want to actually comission when I am eligible to do so. Your final select letter is not a binding contract, but will contain the date at which you are eligible to commission. Of course, when you receive your final select letter, you will not want to tell your recruiter that you are unsure as to whether or not you will accept it. If you decide for certain that you do not want to accept it then go ahead and tell him, so that someone else can have your spot. You have to do whats best for you, even though he may not like it if you back out at the last minute.
I spoke to a few people that have done NHSC, and they personally recommended doing HPSP instead. The general sentiment was that you will get sent to an indian reservation, or an inner city where people are not very accepting or appreciative of what you are trying to do for them. Take that with a grain of salt because those anecdotes are from a pretty small sample size. I am young, not married, no kids so I figured HPSP would be a good experience.