Category Membership And Hierarchy - Best Practices/seo Implications For Sub-Categories

I'm wondering what the SEO implications are when setting up sub-categories. Can placing sub-categories under a main category diminish their visibility? We have five important categories which are all various types of our main product, for each of which we intend to do complete SEO. The more generic main category would logically have less SEO information that applies to all products in its sub-categories, unless we treated it as the union of all it's subcategories rather than the intersection. From a site navigation POV, the main category does represent the union of its sub-categories. I would think it should be set up for SEO the same way.

In a category hierarchy should the most specific category or the most general be checked as the main one in products? If several categories are placed under a more general main category can the main category be set up to inherit the SEO properties of all of the sub-categories or does this need to be done manually?

We could have15-20 categories, including sub-categories. Consolidating our five most important ones under a main one would be natural, streamline main navigation and also give us expansion room. One drawback is it could put our less important and sparsely populated categories on a par with more important things in navigation. Also, I'm wondering about SEO as mentioned above.

Would appreciate comments and/or recommendations.