Are there any basic rules to upholstery?
Rules are merely guidelines about what works in many cases. (Also see The Basics) The important thing is how the job turns out, not whether or not we followed a rule. The quality of the finished product is more important than the method used to produce it." You use whatever methods, skills, tools, and supplies needed to produce a quality product.
For the sake of clarity, I will use the word "guidelines" instead of rules. Over the years, like any trade or profession, upholsterers have developed different practices and methods that work best in many/most cases. It is much easier to do jobs when you follow one's normal practices. However, (again like any other trade) upholstery cannot be narrowed down to just following the prescribed practices. The upholsterer has to always be aware when other methods may work better than his "regular" methods. It has been said that (something like) "if you don't know how to do something in at least several different ways, then you really don't know how to do it well."
Over the years furniture changes, the tools and supplies available to the upholsterer changes. The expectations of clients change. The upholsterer must adapt to new types of projects and to use his skills to tackle many new challenges.
In the ever changing field of upholstery, more important that the upholsterer's skill is his willingness and his ability to learn new skills and methods, to adapt and change to be able to tackle whatever projects come his way.