An old plumbing contractor I use to work for up in Chicago, told me some numbers when we roughed-in plumbing fixtures. For a mixing valve/shower valve for a tub was 36" from the sub floor, or 40" for a shower. The tub filler (spout) stub out , 4" above the tub deck. I can't quite remember the exact height he wanted for the shower head stub outs. Now I'm 6'5" and I would stand in the tub and put the stub out right about my head height. With the average shower arm and an average shower head, it usually dropped the level about 4-6 inches. On all of my installations like that , I never had a complaint that the shower head was too low. As for the original question, it appears that there is no actual required height for a shower head, I always keep in consideration, especially during the rough phase that the stub out, and the finally height of the shower head itself, will be a good 4-6 inch difference. Frank