Hi as a iFly tunnel instructor I teach first timers very differently then someone who is a skydiver or going through AFF.
For first timers AS LONG AS THEY HAVE THEIR HIPS DOWN having arms and legs out is very stable for JUST flying around in a small tunnel. It is not a good position if you want to learn to turn, go up and down, pull, or other maneuvers. But I don't want first timers to know how to do maneuvers before they know how to be safe in there.
As stated above we keep first timers flat to keep the wind speed low. This is for safety of both the student and the instructor. I don't want first time students to go above my head because i can't communicate with them and honestly i don't want to have to catch someone from that high. You don't have to teach them this way but most do because of the safety of it all.
Also we are usually teaching a group of people so we have to come up with a way to teach the entire group something easy for them to remember. Students will forget everything you say to them between the class and the time they fly. I try and keep everything as simple as possible. They are here for an experience if they want to learn to fly they can come back for coaching and I can teach them proper body position.