I have a deep belly button so I dip a q-tip in hydrogen peroxide and clean the inside of my belly button at least 4 times a week. If you have a innie bely button and don't do this, your belly button probably smells like rotten cheese.
Also, if you just put soap in there and wash it like the rest of your body, chances are you won't be able to dry it as well and will end up with a musty nasty belly button.