I reckon you can teach it either way after all power+pitch=performance. either method will work as you are simply manipulating the aircraft to get it to do what you want.
however for single engine a/c it is probably more consistent to teach pitch for speed because at some point you will need to teach them PFL's, glide approaches and EFATO. Much easier to do these exercises i think if they are already using pitch for speed.
just my very humble opinion..
