Cana anyone recommend a plastic wrench that doesn'e scratch the metal?

Hand tight is fine. For a bit tighter a wide rubber band will greatly increase grip and leverage. If your binding posts are poorly designed or you're really determined to overtighten then tape your metal wrench. Sorry but people who overtighten are a pet peeve of mine. Don't be that guy. Please.

Over-tightening could potentially cause a problem when you go to remove the spades.  Binding posts are typically mounted to the cabinet/plate using a nut on the backside.  If the spades are too tight, it could cause the back-nut to loosen and then you are stuck with a rotating binding posts with spades that are locked on.

I wrap some blue tape over the ends of a wrench but as millercarbon stated DO NOT overtighten!

Once you know what size you need go on line to Snap-On and order the appropriate sized open ended wrench. Their wrenches are entirely polished and chromed. They will not scratch anything. Just be prepared to spend a bit more on them.