No matter how small your dog happens to be, his teeth need to be brushed on a daily basis just like your own teeth. Think about it for a second--how bad is your pup's breath when he goes to "kiss" your face? If it smells like last week's dinner, then it's up to you to do something about it. Although dogs are lucky enough to not have to worry about cavities the same way we do, they still have to worry about gum disease, which is where you, a toothbrush, and some dog-friendly toothpaste come in.
Your dog probably doesn't care what his breath smells like or if his teeth are yellow. The only time he is likely to worry about oral hygiene is when his gums become infected and painful. The best thing you can do is start brushing your pup's teeth at a very early age. This will help him get used to the brush before he has time to be upset about it.
The earlier you start brushing your pup's teeth the better. Young pups are far more accepting of things like this, making the process much easier as your pup grows older. Brush them daily to help ensure your pup's teeth and gums remain healthy for as long as he lives. Can you imagine your dog with false teeth?