Aggregate throughput in multi-hop ad-hoc networks suffers from the presence of hidden and exposed nodes. While there are various approaches to mitigating these problems, in this work we focus exclusively on the role of physical carrier sensing (PCS). Specifically, tuning the PCS threshold impacts the `exposed' and `hidden' areas differently; increasing one typically decreases the other, indicating a trade-off. In this work, we first develop an analytical model (assuming constant rate nodes) and formulate an optimization problem to determine the optimal PCS threshold. It is shown that under some conditions, the carrier sensing range should be chosen to equal the interference range. Numerical results with MATLAB and simulations with OPNET are used to verify the optimality of this result. Finally, we extend the analytical model to more general scenarios - random networks with multi-rate - for optimizing aggregate throughput.