It's probably worth making a clear mental distinction between labeling and tagging.

Labeling is, imo, saying 'such and such an author writes x type of book'

Tagging is a required part of e-marketing. It has sod all to do with sticking people in niche boxes and everything to do with ensuring that when a reader searches for something good to buy, you pop up in their list.

If possible I'd recommend embracing (and exploiting the crap out of) the latter without confusing it with the former. If a writer wants their website and their work found, then they need to tag for every classification they could conceivably fall under.