IMO it's only sandbagging if one or more of the following applies:
1. You won a major award (Best in Class or equivalent) with that costume at a large event, and now your'e competing at a smaller event.
2. You won an award with that costume at a higher division and are now competing at a lower division.
3. You won Best In Show and are now competing the costume again at an event of equivalent or smaller size.
Best Prop is totally fine IMO, as are other minor awards. People often compete the same costume a couple times in the same year, starting with a small local con as a 'trial run' to work the bugs out and then progressing upwards to bigger competitions. Once it wins big at a large con (or once it's accumulated a bunch of smaller awards) it's usually considered that the costume should be retired from competition unless major modifications/improvements are made.
COSTUME-CON 32 - 2014
the den of sarcasm - my website
Magpie Creations - jewellery and wa-loli accessories
The International Costuming Convention is coming to Toronto Canada!
4 Days! 4 Masquerades! Dealers, Tours, Parties, Costuming Panels and Classes, Cosplay, Props, Doll Show & Competitions, Steampunk, Sci-fi, Fantasy, and so much more!