EndUserSharePoint.com: Best Practices – Place a site column as high up in the food chain as possible
Best Practice: Create site columns as high up the site collection food chain as possible, preferably at the top of the site collection.
Why: If you are finding the need for that column, chances are so will other people. With a large site collection, it’s easy to duplicate efforts if things aren’t managed in a central location.
When you don’t have access to the site collection level, it’s worth a discussion with the site collection manager to see if they will do it for you. You’ll be surprised at what a couple slices of hot pizza will get you. — Mark
The only time you can’t do this is when the list holding values for a lookup site column is not in the top-level site collection. This happens when I’m using site structure to control permissions or when I want to “localize” a lookup (e.g., Department Issues).
Paul – agreed.
If a site collection is structured with some forethought, any lookup list that will be applicable for multiple sites within the site collection should reside at the top level of the site collection.
Why would you do this? Using that lookup list as the basis for a site column now makes that site column available to the entire site collection.