I take in consideration the doughnut shaped radiation pattern of the antenna, but even so I prefer to keep it horizontally,avoiding the twisting and bending I may inevitably do to it before and after each flight. Inside the antenna case there is a thin cable that if break by fatigue will inevitably crash your plane.
Doing this since day one.. and not a problem whatsover in 5 years of flying 2.44 Ghz, even in crowded RF enviroments.