In the EU short stories he takes it off while in-flight. He also reflects, in thought, in one of the stories that from time to time he'll rely on pretending to take the helmet off because it creates such drama in his audience that he's able to get away with almost any subtle attack. Case in point, he moves to take the helmet off as a point of tasting some spice he's supposed to deliver and while his host (and his bodyguards) watch apprehensively he's able to toggle a poison release on his belt.
Good stuff...
And by the way, since I'm making an ESB inspired EU costume, I'm going without jet pack...
The original reason was because I don't want to have to worry about it, and Fett would likely be wary of wearing the thing that nearly killed him in ROTJ.
But ultimately the best reasoning is that by 20 years after ROTJ he has half an artificial leg and two bad knees, he couldn't stand the impact of landing.
Phil