There is no reason to keep things around that you aren’t using anymore. Read this answer to a recent question to learn how to inactive invoices you are not using.
Q: How do I set my custom invoice to be the default invoice that comes up when I go to enter a new invoice? There is one that I never use and it always comes up as the default.
A: An easy way is to inactivate the invoice templates you will not be using is to click on Edit > Template and then, right-click on the template and select Inactivate.