I think it depends on what else you have in the queue for your clients. Without knowing what other priorities you have, I can't say whether this is more or less important. However, as Patrick said, correct markup helps search engines understand more about the context of what's on the page. It probably won't make a big ranking impact, and I doubt you're going to get penalized for some markup errors as long as you're not trying to spam using markup.
I definitely prefer Schema.org markup over Hcard, but rather than implementing it as per Schema.org you may want to consider JSON-LD - https://developers.google.com/schemas/formats/json-ld .