No, Free spins in a slot machine bonus are not pre-determined. Each bonus spin is a random event of the random number generator.

Certain games such as Dancing Drums, allow a player to select how many free spins they would like. For example you can select less free spins with more ways to win, or more free spins with less ways to win. What this is doing is placing you the player in the driver seat to select how volatile you want the bonus round to be. The option with less free spins is the risker bet but has the potential to pay out a lot greater on line hits. The option with the most free spins is the safer bet, but you are giving up the potential for greater line hits. Use your remaining budget as a deciding factor with games like this. If you are up on your visit and want to try a little more risk, you can opt for the least free spins with more ways to win. However if your budget is limited, you may want to play the safer route and select the most spins.