In the literature, it is common to consider that sensor nodes in a clustered-based event-driven Wireless Sensor Network (WSN) use a Carrier Sense Multiple Access (CSMA) protocol with a fixed transmission probability to control data transmission. However, due to the highly variable environment in these networks, a fixed transmission probability may lead to extra energy consumption. In view of this, three different transmission probability strategies for event-driven WSNs are studied: optimal, fixed and adaptive. As expected, the optimum strategy achieves the best results in terms of energy consumption but its implementation in a practical system is not feasible. The commonly used fixed transmission strategy is the simplest but does not adapt to changes in the system's conditions and achieves the worst performance. In the paper, we find that the adaptive transmission strategy, pretty easy to implement, achieves results very close to the optimal one. The three strategies are analyzed in terms of energy consumption, and cluster formation latency.