I would never advise students to focus mostly on grammar, but spending some time focused on grammar can be useful (Paul Nation recommends spending 25% of time focused on language, including grammar and vocabulary, for example). Also, most students want to spend some time learning grammar and improving their grammar so they can speak and write more accurately.
All of the students in my classes are at different levels and have different needs, so they need to be able to address their needs independently, outside of class. What strategies / activities can I teach them so that they can improve their grammar independently?