Electure,
Darned good call mate.
As an ex-EC, I used to have this conflict of what I should do and what my Boss would have me do.
As the Installing Electrician, the primary Inspection, matters the most.
It is entirely up to that Electrician here if they want to connect into existing work.
The up-shot of this, of course, is if the wiring that you "tap" into is bad, and it causes a fire, it's on your head.
Personally, I run a circuit back to the panel, each and every time.
I don't want to have to take on any more liability than what I should.
IMO,some Electricians these days don't seem to take into account the effects of things like Voltage Drop and the length of circuit run as it applies to Protective Devices.
These are the basics of the trade.
