Dave, in my experience it's tank to tank.
Monti caps tend to be fairly hardy, and besides overlighting, I've found them to react well to tanks that Acropora might not thrive as much in ... yet.
But it's tank to tank, and I'd tend to think having `previous tank' rock might help, as well as more experience keeping water quality high never is a bad thing. [like CAReefer's point]
While just a general guide, I tend to like the `when you have coralline grown as big as a quarter, Acropora will likely grow/do well' ... which is like the 6-month idea, varies a bit tank to tank.
If I set up a new tank again, I'd wait 6 months to start stocking frags [would add Pocillapora and Montipora a few months prior] ... and not really get gung-ho about Acropora until after a year. Figure I saw so little growth, mostly brown the first months with Acropora anyway ... but that's my one anecdotal experience, take it just as that.
Lots of wise advice in this thread ...