I agree with CognosProfessional, it depends on who is using the catalog. We have the data separated into folders by subject area and within that folder separate the facts from the dimensions. We go another step, as we have multiple subject areas we have common dimensions in a separate folder from subject area specific dimensions. the result looks something like this,
Sales Subject Area.
Sales Facts
Sales Key Dimensions
Product Dimension
Time
Customers
Sales Reference Dimensions
Sales Channels
etc
Stock Subject Area
Stock Facts
Stock Key Dimensions
Product Dimension
Time
Stock Reference Dimensions
Warehouse Dimension
Shipping Dimension
Database Tables (this folder contains all tables and is hidden from normal users)
As we build these subject area folders from numerous database tables we hide the background work from the users. There is a bit of work when adding new fields etc to common folders but the payoff is a much more user aligned solution. The one thing we do stress to the users is that they stay within 1 subject area when selecting fields for their reports due to the way the tables are joined. Eventually we will have all of this controlled via Architect but at the moment it doesn't seem to handle Oracle Materialised Views very well and our 2 largest dimension tables are built this way.
It can take a while to get a catalog set up correctly and might need several iterations to get to a workable solution but is time well spent if it helps in the take-up of the system by the users.
Good luck
John