I have seen airlines discourage that. I wonder whether it makes things better or worse (or whether it is some sort of prisoner's dilemma - making it better for yourself, but worse for others?).
Really? Admittedly I only fly Delta, but they've certainly never discouraged it. Also, I don't really see how it could make things worse for others, since it would also be pushing any viral particles you may be expelling towards the floor where the intake vents reside.