I have found some signs to be more necessary and useful then others. The first one I ever taught him was all done when he started on pureed foods. Whenever he finished he would sign it, but I am not sure he actually grasps the concept of it as in 'I'm full' vs 'the bowl is empty.' It's more obvious when he is done eating because he will push away his spoon and hold his mouth shut.
He also signs for more when he is eating, and this one has been pretty helpful and has forced him to communicate with us while he is eating. It's pretty funny to watch him when he eats something like ice cream or cake, because he pretty much signs for more as soon as the food his his mouth.
The most helpful sign I have taught him so far is the one for milk. The other day I was in the middle of a store with Noah and he was getting really fussy. I wasn't sure what he wanted so I literally just said, "Noah, what do you want?" and he looked right at me and started signing for it. So I gave him his bottle, he drank it up and was then perfectly happy. So awesome!
The next one I plan to teach him is for change, as in diaper change! We'll see how it goes.
I took a video of Noah signing yesterday and planned to post it with this blog, but then I realised that it's actually on my mothers camera at her house. Will try and post it on facebook at a later time. In the meantime, here is a video of the first day or two of teaching Noah how to sign more. Gives you an idea of how it all works.
I definitely plan to keep this up for my next children, and if you are interested in early communication with your child I'd would totally recommend it :)