Use dashes or underscores when naming documents?

We currently use underscores (_) to name our documents, but the "standard" seems to be dashes (-).

What are the prons and cons?

Personally I prefer underscores because they read slightly better, for example:

how_to_install_gitlab_on_your_washing_machine_and_let_it_worry_about_drying

versus

gitlab-is-awesome-as-it-integrates-with-everything-even-my-wachine-machine