I always have more percussionists than I need. Sometimes I might have my extras help count rests or something during the explanation, but they always just stand/sit against the back wall during the performance. I've done this in several regions and have never been questioned.

If you don't have all of the percussion parts covered, that might be a different story. You might be questioned why your "extra percussionist" is not playing the available part. In that case I would definitely email the region secretary for clarification/CYA.