Unless you are an experienced traveler, I would strongly suggest you deal with a real cruise agent. Yes it may cost you $10 to $50 dollars more, but more importantly they may save you from disastrous vacation.
Some ships overbook, do you want to be the one bumped? The cruise lines don't want to lose agent's business. However if you just booked online, you don't have any real 'bite'.
Room not up to standard, missing your bags (airline or cruise line), ship late arriving due to weather... a lot of things can happen. A cruise agent can be your greatest friend.
I've been on ships where I've watched people get down graded and even bumped, they were not 'happy campers' to say the least.
If you have cruse experience or heavy travel experience, than go ahead and use the online services. If not, contact a well known cruise agent (not travel agent).
If you need assistance in finding one please feel free to email me. There are many good ones across the country.