J. L. Pomeroy has flown long enough to know that there are limits to how obliging she will be to her fellow passengers, especially those who want her seat.

“I always pretty much just say no,” she said. She rarely gives her decision a second thought.

“I always pick a very specific seat for very specific reasons, and somebody’s fleeting whim at the last minute isn’t something I want to accommodate on a long flight,” said Ms. Pomeroy, who runs a film and event production company.

For passengers in increasingly stratified plane cabins, the scramble for the right seat has become more intense than ever.

Just asking to switch remains a popular choice, but increasingly, frequent travelers say that fellow passengers are breaching long-established etiquette and simply plopping down in a seat of their choice.