I don't recommend billing the total amount for the year up front, unless there is a bona fide accounting reason to do it, such as wanting to record the revenue (or deferred revenue) in month 1.

If the reason is to save on data entry effort, then Doug's suggestion about copying invoices is a better option.

The problem with billing up front is that making adjustments is extremely painful, and customers are always making changes throughout the year.

One thing Adagio can't do is to have multiple due dates on one invoice. It would be a really nice feature - I have some private school clients whose tuition (and other miscellaneous fees) is due at set times, and they can never get a true overdue aging.