Modelling complex systems such as multiple-use reservoirs can be challenging. A legitimate question for scientists and modellers is how best to model their management under uncertain rainfall. This paper studies whether it is worth using a stochastic model that requires more effort than a much simpler deterministic model. Both models are applied to the management of a multiple-use reservoir in southern Vietnam. Although no single modelling approach is universally superior, this study indicates that the desirable modelling approach is stochastic if reservoir capacity and water use demands have a high enough impact on the optimal timing of reservoir water use.