Great Question and even more difficult to answer if you collect ALL things Soulful.
Way back (35+ years ago when all US issues were rare) I always had them alphabetical by Label name (Action, Atlantic, Beacon, Bell, etc.) but, although Label name is still the main filing order, the collection is now divided into US (60's & 70's) & UK (60's & 70's) 'Top level' sections and then by label with seperate drawers (and boxes) for such sections as Deep Soul, Motown and some by certain Key artists (Womack, Dees etc.)
Works for me.. but, certainly, only I would be able to find my way around it.
If you just collect 'Northern' or 'British' the task has got to be a hell of a lot easier.
Always perplexed me how others do it though!
Sean Hampsey