Forever on. They're not just for decoration. They provide absolutely essential information surrounding gravity areas, fuel scooping and angles for approaching star bases. Using those rings and lines, you can save incredible amounts of time in your approach to ports and allows you to better see dark planetary bodies with ease.
If you enjoy wasting your time, by all means turn the lines off. I agree they are not especially attractive. But as a pure matter of efficiency, visual clarity, and in some cases SAFETY, having them on provides more than enough advantages to justify it being enabled. Some of you may not realize just how much time you could be saving because you're taking poor angles and approaches to ports anyway. When you better understand how to approach ports for maximum efficiency (right angles), you'll realize that having lines on is the very best way to do it. And maximizing efficiency doesn't just save you time; it could save you from an interdiction that could have otherwise taken your life.