You live and you learn, and your ideas about things change. Such is the case with fill lighting for me, a subject that I explored pretty thoroughly in the Topics section of the Portraitlighting.net site. Today I'll mention two deficiencies in that treatment of the subject.
The first came to light when a commentator, on explaining the position of his fill light, said "it's perfect here, if it were any closer it would act as a main light." That statement was seminal for me. The concept was so obvious in retrospect, but somehow it hadn't seemed that important to me. Put another way, if the fill light is too close to the scene, the effects of the inverse-square law will be clearly seen, and the fill light will look like another main light. This phenomenon is seen quite clearly on the website in Example 1.3 www.portraitlighting.net/example1_3b.htm , but as it applies to the main light. In the second image in that example, the light is further back and more appropriate for fill. At that distance, there is a more general and flat sense of illumination, without the added sense of depth and form visible in the first image. Now that doesn't mean the fill light has to be further away than the main light for all applications, but for classic portraiture, where a subtle and indistinct fill is usually desired, it's a good thing to keep in mind.
The second revision relates not to position, but to number. When I wrote the piece on fill lighting, with a few exceptions, fill was considered from a single source. While it is probably true that most portrait studios have one fill light, if they have any at all, that doesn't mean having just one is best or even the most natural solution. After all, most natural-light images receive fill lighting from multiple sources and from multiple directions. Forty years ago, when softboxes and umbrellas were still rare, and large parabolic reflectors quite common, a single fill source was often not the best solution. In order to create an indistinct fill light, some photographers bounced the hard light from multiple parabolics against neutral-colored walls and ceilings to create a soft enveloping fill light. This approach created a fairly uniform fill illumination throughout the camera room, with the added benefit of no distinct fill-light shadow or catchlight in the eye. While this technique has largely been abandoned and is expensive to implement, it still has its place. It's a technique that works well for environmental portraits, especially if you have a bunch of speedlights and the walls and ceilings are reasonably neutral.