I don't know if I have enough experience or knowledge to give you a definitive answer, but here are a few ideas.
First, I assume you are keywording your personal photos. They are to help you to find them later. That's as opposed to, say, a stock photographer who keywords so others can find (and buy them). Or a corporate photographer who keywords so others in the company can find them.
Second, I think the first thing to consider is how you anticipate you'll search in future. Think about the kinds of searches you'll want to do. For example, I photograph a lot of vehicles. I want to be able to find all photos of a Bangkok MTA Hino AK bus on route #2 on Sukhumvit Road. Or all photos of a British Airways Boeing 747 at San Francisco Airport.
Knowing the kinds of questions you want to ask suggests the keyword structure you need to build. I effectively developed my own "Controlled Vocabulary" for keywording. Some serious keyworders purchase a controlled vocabulary. I think my needs are unusual (Ok ,weird) and I am frugal so I developed my own. I entered many of them before I keyworded a single photo and made sure that the organization, using Lightroom's hierarchy, was general enough to expand.
So, for example, I want to be able to search down to the street level in major cities. I have a keyword hierarchy that reflects the organization of Bangkok as that is my home. But I also made sure that it works for cities I visit, for example London, Hong Kong and Singapore.
Similarly for vehicles I developed a hierarchy based on vehicle type, manufacturer and model. For buses and coaches I developed another hierarchy based on operator and route identifier (number).
And that raised an interesting little issue. That is that keywords are "typeless". So it isn't possible to tell from a photo that has the keyword "3" if it is of a "Mazda 3" car or a bus on route #3. Indeed "3" appears in many places in my Lightroom keyword hierarchy and sometimes confuses it no end.
I developed some workarounds for that using keyword synonyms to add a notion of type.
Having typed this I have no idea if I am on the right track to helping you with your needs.
Let me know if this in any way fits the way you think about your photo collection. I've written a lot about it on my blog so feel free to take a look if you are interested. For example the post on My Workflow : http://bkkphotographer.wordpress.com/2009/09/19/my-workflow/
describes my keywording in a bit more detail.
Cheers from Bangkok.