[avit]

I think this depends a lot on your invoicing style:

Do you only invoice when a project is complete?
Do you invoice at regular intervals?

If you invoice each project immediately when finished, then it makes sense to put the task on the project.

If your invoicing is done at regular intervals, then I would say make a task for "Invoicing" that repeats every 2 weeks, and do all your projects as part of that task.

If you don't have a separate system for tracking unbilled work for invoicing (i.e. nothing to remind you that a project needs to be billed) then it might make sense to put an item on each project as well, regardless of whether you use a master task for it.

Also, individual tasks for invoicing each project should be more manageable chunks, depending how complex your projects are. I find that a single "Invoicing" task can look daunting when you know it will take you a couple hours to work through it.