Have a look at this list: http://codex.wordpress.org/Plugin_API/Action_Reference
- It doesn’t matter which one you use as long as it’s not too early and not too late. It’s best to use intuitive and predictable hooks, so
add_meta_boxesis preferred. Someday in the future WordPress may change something and by using the most appropriate hooks you increase your chances that your code will still work in the future. - There is one exception that I can think of to that. Sometimes (in cases that are very unlikely to happen) you may need to for instance call
add_theme_support()which is generally used withafter_setup_themeaction hook only for logged in users that are administrators but this hook doesn’t allow you to access this information yet. You would therefore have to probably useset_current_userorinitaction hooks instead (after doing some research if it’s safe to do that).