You can add it after booking, but if you have a room-only reservation you must convert it to a room plus tickets plus dining package unless you're an annual passholder (AP holders must convert to a room plus dining package).
You can add a dining plan to a package reservation up to 72 hours before arrival. You can't add one after you arrive.
If you have a DVC reservation you can call Member Services and add a dining plan up to 48 hours before arrival.
Whether it's a good deal or not is going to be entirely dependent on where and how much your party eats (do you go to a table service restaurant every day? Do you prefer signature restaurants - those will cost you 2 credits per meal instead of one), what they prefer to order (such as - do you want dessert for every meal? Do you like appetizers - they're not included with the basic dining plan and the deluxe plan, which includes them, is a lot more expensive.)
If you're unfamiliar with the three different Disney dining plans, check the "Everything you always wanted to know" sticky thread for an outline of what they cost and what they include. Keep in mind that March is peak season and if you choose the basic dining plan in that period it will cost you more than it would ifyou were traveling off-peak.