It is pretty common with a sex sling to use a stand. Many people are just not willing to put four hooks in their ceiling. If that is the case for you, I would recommend you take a look at our Stands page. Of course, if you are OK with four mounting points or you have a dedicated room for these types of things, a direct ceiling mount is a great option.
For a sling, most things are a general guideline, and there is no fixed answer. Your mounting options are limited in one direction by the spacing of your ceiling studs. In the other direction, I would aim to be near the higher end of the recommend ranges because it will be more comfortable for you.
If you have a leather sex sling such as those from Trinity or the Stockroom, I would recommend making the mounting points at the base wide because it will be more comfortable, but for the top of the sling, aim for narrow. About the width of the sling. This is because the way the straps at the top are attached if you make it wide, it will put a lot of pressure and twist in the straps. To me, it just does not look good, and why put all that load on a couple of rivets.