Pretty much, it is quite hit and miss. I do the same method as what Liam said, by double tapping shoot. The double tap is for all vollies i find. It's for if you don't want the player to perform a header, double tap will find the shot/animation available. If there is no volley animation available i think they will just try the header still.
And as for positioning, have only ever done a proper bicycle kick (as in not the one where he falls backwards and kicks) twice in PES5. Once in a game where its bounced off a defenders head and was pretty much directly above my player - double tapped and he attempted the overhead. And in training i got it once from a ball played into the box, player chested it up to himself, and then scuffed the overhead. Have done the small crap overhead a few times in training, never scored on in a match though. It's basically all about having the ball over ya head, and double tapping. (well thats what i find anyway, but like i said.. i've only done it a couple of times - and there have been times where it's looked perfect for one, but they've just headed it).