Thanks. Only 2 levels really, born out of a desire to not have to code the same menus, banner and footer for every type of page or change all of the templates when we update those parts (as we did about 18 months ago -- the site no longer looks like it was designed in 1995!).
We currently have three different base-level templates and about 24 second-level templates (i.e. children of one of the three). It hasn't been well managed, since we could have replaced much of the basic stuff with server-side includes or ColdFusion files -- got part way on that project and stopped.
We will eventually be moving to a CMS which will solve lots of problems, but we're having to invent our own modules to fit in with the proprietary CMS everyone else in the division is using and that's slowing it down.
So far the best temporary fix seems to be to insert an editable region inside the existing "head" region in the secondary templates. As a TemplateEditable region, it does carry through reliably to the child pages, which means we can embed custom style modifications when we need them. I just have to do it to all the active secondary templates in a text editor. I did try one and it worked splendidly.