I'd disagree that its firmly established.... but it has been firmly established that two Warp nacelles is the optimal configuration for a starship.
My beloved copy the first Star Trek Technical Manual (circa 1975) had single-nacelled ship configurations.Of course, this isn't the first time I've seen single nacelled or triple nacelled ships in Star Trek shows, but it drives me nuts everytime I do. I believe there was a single nacelled ship in Star Trek: TMP.
'How things are supposed to be' varies wildly in the Trek universe(s). There's not as much continuity in the continuity as we were led to believeI just wish there would have been a little more effort to stick to how things are supposed to be, instead of making changes just to make ships that look cool.

FYI, I was watching the new Blu-ray of The Wrath of Khan last night. Scotty keeps referring to the main 'energizer', which apparently powers the ship. First time and last time I recall hearing that specific term (wasn't it called a 'matter/antimatter reactor').