I think you mean "I would not encourage..."I would encourage unless you want to exaggerate or overbuild IR. Palm up in a quality pitcher usually indicates a riseball or palm up curve.
It depends on the level of the kid. I agree up to around 18U, palm up is not that a big of deal. Young pitchers have a lot more to work on then "palm up at 9".
When a kid has the motion down pat and want to be a good D1 pitcher, having palm up can add 3MPH or more.
My own DD's journey with palm up at 9 was that she was learning a palm up curve. She never could get the curve to work, but she found that palm up added 3 to 5 MPH onto her speed. She incorporated into her motion.