Language technology resources are limited for signed languages. In order to improve these technologies, large amounts of labeled language data stored in annotated corpora are necessary. Many existing annotated corpora in American Sign Language (ASL) do not include non-manual markers (NMMs), i.e., language features that do not involve the hands. This work underscores the importance of including NMMs in sign language research by describing their plurifunctional nature in signed conversation, specifying their link to sign language prosody, examining the current state of ASL corpus research, and recommending a systematized method of annotation which captures the critical but complex variations in NMM production in ASL.