You only have to install the SSNB ONCE on any URL you register. Whenever you join a ring, you will be prompted to add another piece of code. You don't need to, though.
Rest assured, both will work as long as your code's U# matches the U# of your registered URL. If you want to know more about U#'s, read on.
What's a U#?
If you look closely at the code you are provided at the end of joining a ring, you'll notice that the code isn't an exact match of what you've installed on your registered URL. Don't worry about that. The difference lies in the "U#".
If you got your SSNB code from your My Account page, the U# will be "defurl" or something similar. If you got your SSNB code at the end of joining a ring, the U# will contain a series of numbers. The numbers are specific to your membership within the specific ring.
The important thing to remember about NavBar code is that the SSNB version is dynamic. This means it can be changed from a distance, so you only need to install it once... no matter how many rings you add your register URL to.