I had a feeling you'd post this after some of the topics that have come up over the past fortnight.
Consider a categorised folio that distinguishes between the types of web design that you do. Something along these lines:-
Custom Web Design: Websites designed from scratch, no templated assets.
Budget Web Design: Business websites based on customised templates.
Joomla Web Design: Commercial websites using the popular Joomla CMS.
The idea is to be forthright about the fact that you've used templates, but in a manner that's positive. Your customers will probably not care, they're focused on how your sites look and work. Your peers will care and if you try to pass off someone else's work as your own, they can jeopardise your reputation via negative PR.
As an aside, I know you prefer to work with templates and you say that you heavily customise them. 'Heavily' is a relative term. Would you be willing to show a template and the completed design for us to gauge the difference?
If the design is as heavily customised as you say, it would be perfectly reasonable to call it a custom design inspired by a previous design.
