I hope you love Virgin Voyages! Of the three cruise lines I've tried so far (Disney, Virgin, Princess), it has been my favorite for food, cocktails, and coffee.
Typically, booking early is the best time to book. Right now, Virgin has an extra $600 in bar tab (for booze), and with certain travel agencies, you can get extra Sailor Loot (onboard credit used for anything). If you do book early and the price drops, you can always cancel and rebook, but you would need to see if you are losing out on any perks by doing this.
This is one of my latest blog posts about Saving Money on Virgin: Save Money on Virgin Voyages