No it does not affect your pictures. Battery grip only serves as an extra power source for longer shoots. It also provides the extra grip and it makes the camera looks bigger thus the more "pro" look... hehe
Do you feel like the camera is too small in your hands? The grip can add an extra bit of comfort for some people making it easier to hold. Do you plan to get other lenses? It will help balance large lenses (aka 70-200 variety). But, this is all subjective.