Attila,
Rules is the wrong word, it suggests that one would be breaking such if they are not followed exactly.
Displays should be arranged based on sound artistic priniciples, the basics of which can be found in any successful work of art. To ignore these priniciples is futile, they work, they have always worked, they always will work. From the earilest pieces of art to those being created today, these same basic prinicples can be found over and over again. Take for example, the golden section, a priciple that, knowingly or not, is used for successful placement of trees in pots, to placement of objects in a display area. Why? It works. One is not required to consider such priciples, but it is rare indeed to find something that works without them.
Display is not subjective at all, negative space, color, direction of movement, flow of lines, seasons, etc etc etc all must be considered for a display to be successful. There are endless cominationsof these priciples that could be arranged, but in the end, the successful display incorporates them.
Will