I've used the Flagship check-in several times at MIA and LAX as of late and must say it's very convenient. I'd say probably best to let them lead you to the front of the non-precheck line in MIA. LAX, that's no issue as they just take you up the elevator to precheck.

Everyone's been polite & professional & I've had no issues gaining access by simply flashing my BA Gold / Emerald card (physical wallet card). A+++ service that can save lots of time over the priority lanes at those airports.