You're asking 2 different questions here.
"When to use a web design template?"
Whenever you want to.
The real question of your debate sounds more like:
For a design to be "truly your own", can you use a template or do you have to design from scratch?
Deriviate works modified from from their templates may be legally considered your own however, but that's stipulated if you're allowed to do make any modifications or not too.
But from a purely artistic perspective, nothing is "truly your own" unless you do everything yourself.