Vertical and fully extended is always best, but not always the most practical. Having said that, if you're sailing a 1/72 scale puffer on a paddling pool where range isn't a problem, the placement of the Rx aerial isn't going to be critical. Just make sure you keep the aerial wire away from motor and servo wiring.
My advice is to try a range check with the most practical aerial installation for your model and see if it's adequate. BTW, I always do range checks with the motor(s) running, as this will highlight any sensitivity at extreme range (that's the radio system - not me!).
Works for me, but suit yourself anyway.
