Thank-you to Terry for taking the time to reply.
FWIW ... in the end I decided to go with one source per Register (I created a new Source Type for Parish Register - Online Images). After having done some ad-hoc lookups for Baptisms, Marriages or Burials where I already knew the date, I am now researching one Register at a time and compiling detailed lists of all entries that relate to surnames I am researching.
Seeing as Essex Ancestors is neither a database nor an index, it seemed logical to treat each individual Register as a Source. I guess with many similar cases, it's "six of one, half a dozen of the other" - you either spend more time up front creating sources, or have to repeat some data in citations.
Anyway, for anyone else using this great resource - that's what I decided to do!