cobra_king is the one to ask about this. He knows. Personally, I never would. The pari-mutuel nature of wagering and the high hold make it an extremely difficult thing to overcome. There are no "off-market" numbers in horse racing. I would think any advantage bettors would need to wait until a few minutes before post to make their wagers if they want to employ any type of valuation analysis. Even then, you really couldn't be certain what your bet actually is until post.