Pretty much what the glorious sarcasm-hime said. And those are the main definitions of sandbagging as I've always understood it. I have noticed more cons starting to explicitly put it in their rules, too, after getting complaints or hearing other cons getting complaints.
But yeah, Best Prop is a minor award, a judge's award or honorable mention of some kind, and doesn't disqualify you from recompeting it. If you really are concerned about someone else mistaking you for violating rules, then yes, you can explain to the judges that you won a minor award for the prop alone and would like them to ignore it while judging, but you don't HAVE to. When in doubt, run it by the masquerade director or cosplay staff head of the place you intend to enter, they can say for sure whether you SHOULD ask to have your prop not-judged, or if you don't need to go that far. A director worth his/her salt might even be willing to run interference should anyone question your eligibility, having already talked to you about it and knowing all the important info.
Founder and President, Madison Area Costuming Society, a chapter of the International Costumer's Guild
Strange Land Costuming - www.strangelandcostumes.com