As like everything else in Blender, it depends. I don’t think there is a one prime directive to rule them all here, you go with what makes more organizational sense to you, depending on what you are trying to achieve. The good thing is that there is no right or wrong. You can convert because you are sure you are not going to make any further edits, you could not convert in case you feel you are going to make further edits. Join or not join, same principle. So don’t worry, the more you practice and make things in 3D, the more you’ll pay less attention to doing things one way or another and just take the approach you feel works for you on your specific project.
