What Scott said
Dreamhost maintains a monthly billing cycle to facilitate you registering additional domains and adding services like a dedicated IP address, etc. during the term of your plan.
For instance, if in May you register a new domain through Dreamhost, that billing cycle will reflect the amount due for the registration of that domain even though your "hosting plan" is paid up. Does that make sense?
If you have no over quota charges, and have not had any added services during the monthly billing cycle, each month's bill will just show "0.00" due until your yearly plan payment is due.