Organists should feel free, and actually need, to experiment with the stops and couplers of the instrument at hand and pay close attention to the results (photo).
The player needs to know the sound character of every stop in the instrument throughout its entire playable range, both singly and in combination.
The most flawless technique at the keys of the organ fails in the presence of a haphazard registration -- thus, organ stops, their history, construction, tonal characteristics, and artistic use have always been a critically important study within the larger study of organ playing.
The united forces of the grand symphony orchestra can sound very much the same from one concert hall to another (the same also hold true for the most part with smaller ensembles), but a pipe organ designed, constructed, and voiced to fill a specific office within a certain building sounds only like itself.
Without doing some experimentation in advance one simply cannot know from a stop list to what degree the foundations, reeds, and mixtures of an organ will blend together.
In this connection organists need to trust their ear.
Their ear is their best friend -- it will lead their brain.