We discuss the design of energy-efficient pipelines for asynchronous VLSI architectures. To maximize throughput in asynchronous pipelines it is often necessary to insert buffer stages, increasing the energy overhead. Instead of optimizing pipelines for minimum energy or maximum throughput, we consider a joint energy-time metric of the form $E\tau^\alpha$, where $E$ is the energy per operation and $\tau$ is the time per operation. We show that pipelines optimized for the $E\tau^\alpha$ energy-time metric may need fewer buffer stages and we give bounds when such stages can be removed. We present several common asynchronous pipeline structures and their energy-time optimized solutions.