I have a few OSes. Of them, only one has templates assigned to it-- CentOS 6.5.
For that one, it has an iPXE template and a provision template. The user_data and finish are unassigned.
There's a couple templates I can't delete-- one user_data and one finish template. If I try do delete them, I receive an exception saying that CentOS 6.5 is using it (well, it's a little more cryptic than that, but someone in the channel showed me a command to run to identify the associated OS using the console and it's the CentOS 6.5 OS).

One interesting point to mention is that If i create a "test" template and substitute that as the user_data template, I can delete the old unused user_data template which i want to delete. Once I unselect the "test" template for user_data and set it to nothing again, however, I can't delete test. It seems as though specifying no default template for a given type doesn't remove the previous association.