First and most important: whatever is comfortable for you and flies well is good.
I personally prefer palm forward (perpendicular to disc), but I throw with a 1 finger FH grip so that might be more comfortable for me.
When you load and unload your wrist during the snap of the throw, that consists of extension and flexion of the wrist if your palm is forward.
If you're throwing palm up (parallel to the disc), the load-and-unload motion would be radial deviation followed by ulnar deviation. That seems pretty awkward to me, but again, to each their own.
Palm up vs. palm forward also has some significant impact on release angles. To me, palm up seems like it leaves more room to roll the wrist in toward your body during the release and follow through. When I say "rolling your wrist" I'm talking about the twisting motion that turns a door knob, which can cause OAT.