I always thought it was kinda hard to throw the ball straight. I taught myself several years ago so I could better teach kids and throw BP and my DD wanted to catch so it was win,win,win. My pitch always seemed to have a little tail on it, as Bill said why would you want to throw it straight?
I do think it is important though at young age to teach location. If you call low and outside and your pitcher can't hit that spot more than half the time then why are you even calling pitches. At 10U and 12U I would stand behind the plate and call four quadrants high/inside, low/outside etc. and if the pitchers could not hit the spots I was calling about 3 out of 4 times I would not call pitches what was the point...only thing I would do was call change ups occasionally but I would teach the catcher and they would catch on themselves most of the time.
No matter what pitch you call or what zone you call it the pitcher is going to have to hit a "spot". If you called every pitch down the middle, down the middle is still a spot. You call a pitch on the river, because you're trying to teach them which spots to throw to, even if they can't currently hit that spot consistently.