What is the most secure method of ensuring that clients can only print to defined queues? What happens if you dont do this?

Maintain printers on a private network visible only to the server, then train users to send document to a defined print queue or printer pool

if your networkable printers are on the same network (visible) to your server AND all your users, your users will be able to (and may attempt) to bypass the print service features and send jobs directly to the printer. The recommendation above is to prevent this.