This is what I generally do, too. It always felt weird to me, even as a kid, that we are kneeling…kneeling…oh, wait, the priest just sat down, let’s sit down now for five seconds…and now he’s standing again. I find it better to just stay kneeling and continue my post-Communion prayer through the moment of silence after the priest sits down. Otherwise, I’m just breaking my concentration and I waste that silent time being distracted. I’m sure others approach it differently.
But, as others have said, there is nothing in the GIRM regulating it. As TisBear said, I know there is a diocese or two where the bishop has issued some directive on it. But by and large, most people just follow the crowd. It’s almost like a pavlovian response.

Even if you’re not paying attention, as soon as you start to hear “the shuffle” of people sitting down and putting their kneelers up, it sort of moves through the Church like a wave.