As one of the first people to make a final decision last year (I think it was about this time of year, actually), let me just say that if you know where you want to go, turn the other spots down. No one is going to judge you (including people on an internet message board) for turning them down before April, and no one is going to feel snubbed. Just send a short and polite email informing the DGS that you have chosen another option, and trust me, no one sends back a venomous email accusing you of being a cold hearted jerk for declining. They all know that many of their candidates will have other offers, and that some of them will chose other offers, hence why many schools have wait lists: they also want to know that if you're not coming, they can make an offer to another candidate and have the chance to woo them over--it's a lot harder for them to do that if they don't get in until April 15! Do yourself, the schools, and the people on waitlists a favor and turn down the programs that you know you will not be attending.
Besides, I felt much better after having put in all the paperwork and could confidently say where I was going the next year. It's nice to have it all finished.