The set of NLM DTDs have emerged as a de facto standard content interchange for STM publishers over the past several years. Recently, the ACS Publishing Division has utilized customized forms of these DTDs made public by the NLM to implement XML-based publishing processes for our chemistry-related journals, books, and magazine publications. In this paper, we look at the drivers behind our decisions of whether customizations should be made, and if so, how much customization is needed, to meet the needs of our publication processes. To frame the discussion of the various customizations, we also offer the concepts of a customization level, a customization implementation method, and a customization profile. At the end, we share some of the successes and lessons from our experiences.