Comment on WordPress SEO Categories by Erik.
For example, I have categories for Amish Shops, Amish Woodworking, and Amish Business, which are arguably all more closely related to Amish Furniture. I guess it’s a matter of how focused a page needs to be, and also maybe you can overdo it. And I know as you said there is a balance between being user-friendly and natural and optimizing around keywords and niches.
As you say with 50 categories you wouldn’t want a sitewide widget (I realize I wrote 30 in my reply above but may end up closer to 50 as you suggest); maybe I would simply “batch” categories in related groupings, and use them on appropriately related pages.
Or maybe I am overthinking this bit :)