In reality.... you should be able to win with whatever bet you are placing... FREE or not, because the outcome of the bet is dependant on the Client / Server seed and Slot machine outcomes are determined using a Random Number Generator (RNG) 

We do not know what proprietary software are used by the developers of the casino, but we do know that the 3rd party providers of slots are not influenced by the casino. (So in theory... all bets made on 3rd party games, should be provably fair)   

Now the in-house games might be a whole other story... because those games are controlled by the casino developers.