Personally I've always been wary of multiple bets in one race, partly from hearing people say you're betting against yourself, partly from thinking "If the TAB is taking 15% of one bet they are going to love taking 15% of two or three bets" that's almost half a bet gone in tax. But then arbitrage betting (if I understand it correctly) seems the ultimate form of covering your ******** with multiple bets and it obviously works OK. If betting on one horse is OK and betting on all the horses is OK logic suggests anywhere in between should be workable.