not true. i feel like i can maintain the phrase while switching hands but i was still curious to see if anyone else did this. you say a breath is appropriate, so you're saying it's okay to stray from the timing of constant 16th notes?
I don't know about jmenz, but I think that, yes, it is appropriate to "Stray from the timing of constant 16th notes." But breathing is not rubato. You don't briefly slow down the pulse and then bring it back up to speed; instead you stop the pulse completely, for a very, very brief instant, and then resume it. Doing so will not give the listener the impression that you are playing fast and loose with the rhythm, but it will make the playing sound more open and relaxed, even at fast tempi. The way to practice this kind of breathing is to increase the interval during which you completely stop the pulse, far more than you will do in the end. And then once you've trained yourself to breathe at the spots you want to breathe at, you shorten the interval until it is almost imperceptible.
Take this for what it's worth. It's what my teacher drummed into me about phrasing in Bach, but I am only an amateur.