So, I mostly play Rebel, but I picked up the SCUM Most Wanted set. If I wanted to use those ships for a Rebel team, is it frowned upon to use the SCUM dials with Rebel pilots?

No, used to be a rule dials had to match faction, but that got a lotta hate, so now you can mix the dials as long as they go to the correct ship

100% legal and has been* since it was released.

Just note that while the dials don't matter the you still need Rebel base tiles to go with Rebel pilot cards. If you had one Z-95 Expansion and most wanted the most Rebels you could fly would be two as that uses up all of the rebel tiles you have despite have three Z-95s.

* The FAQ may have said they needed to match for less than six hours before it was corrected.

Oh! Yeah, I knew using SCUM pilots on a Rebel team was a no-no, just wasn't sure if I'd get crap for using a SCUM dial with a Rebel Z-95 or Y-Wing

You may still find people who will give you crap about it, and maybe even a few who will think you are cheating, despite it being 100% legal.

I've always seen the extra dials with alternative graphics as a waste of cardboard space. I'd be so much happier if they had put it more base tiles for the ships even if they are more copies of generics to go with cards you may have or just other pilots who would need their own cards.

You can also get the plastic dial covers and nobody would ever know. I prefer them because you can see the entire dial at one.

No ethical question here. I thought you were gonna reveal something juicy to drive the board crazy - like knowingly rolling an extra die or something.

At the Naboo Open, I was running Thug Lyfe in the Hangar Bay, and specifically brought both Rebel and Scum dials, so that if I were paired against a Scum player, there'd be no confusion.

Yep, no problem.