It's all relative to how you are most comfortable holding the Tx. I like to hold mine so it is parallel to the ground. With that being said, you always want to have the flat plan of the antenna facing your Rx, not pointing directly at it like a spear heading to a target. All that said, I keep my antennas pointing straight out from my Tx and actually tilted down a bit because the way I hold the Tx, this make them 'aimed' up and away from me which is exactly where the Rx is for me and the way I fly. I generally pivot around as the craft crosses over me so I always have a good signal path.
That's my $.02.