Hi ShawnL, anytime is a good time to cruise, but I especially enjoy a nice Caribbean cruise during the later winter/early spring months. Bastically because, I'm from the Midwest and winter can really drag on and on and a cruise is the best thing to break it up. Also I try to avoid going during the hurricane season, which is summer/fall. I think you will find that the temps and weather doesn't vary that much once in the Caribbean, so if there is no threatening weather, you can enjoy the islands pretty much all year round.
It depends on what you're looking for. I've gone in Jan. before and really liked it because there are FAR fewer "little" people! But have also gone at the beginning of the summer and it wasn't all that bad either! Jan. cruises are generally a bit cheaper too from what I've seen.
My favorite time is mid-March. Most of the school & college breaks are over, fewer chances of rain and the island temps are just right. We love to get away from New Englands cold and the dreary days at the beginning of Spring. Its just far enought after the holidays to enable me to regroup and save some spending money and catch a few bargains on new cruise clothes too.