I'm currently using approximately 15 MB of data transfer per month, so I'm not worried that I will exceed the limit.
But just in case that I would exceed the limit, it would be very bad.
I figured out there is a way to prevent that from happen, Bandwidth Throttling.
What would happen if I did not set up a directory called mydomain.com.throttled? Would Bandwidth Throttling not work? Or would visitors see a not found message? Or would anything else happen?