The whole point of teaching sign to babies is not about raising IQ levels or smarter babies, it's about communicating. It's about giving little ones the tools to be able to tell Mom or Dad that they're hungry, that they want to play, that they want some juice, and so on. Infants have control of their hands before the natural development of speech, so they are able to develop signs at an early age. This early communication can be visually measured and seen, which gets everyone so excited about the budding genius before them.
So if you've decided to teach sign to your little one, kick back and enjoy the learning process together. American Sign Language is the fourth most common language in the world. Have fun with it.