The goal of any lighting system is to meet the plants requirements for growth. That requirement is ~600-1000µmols for flower....and even a little more if you got it.
So what ever height meets that is where they should be(then just accept the coverage). Unfortunately, par meters aren't everyday garden tool, so the recommended hanging height stated by the manufacturer should correlate to the requirements. But no guarantee on that.
Cheap and/or lower wattage led's will need to be hung lower to keep up the intensity. While the better led's are just as powerful if not more than any hps. And will need to be at 20"-24" and still meet the µmols(light) requirements for the plants.
LEDs are not a cheaper or less powerful choice to hps...they are just as powerful and better when you get a quality light that will truly meet and exceed the plants requirements.
Apache, A15, illumitex, CLW and maybe blackdog. My 2cents.