Metadata, Default Values and Required Columns
To improve the quality of metadata associated with documents it is advisable to make columns required:

When applied to “Choice” types, the first choice from the list is automatically selected. Thus the user is not forced to make a choice, and the likelyhood is that the correct value will not be selected.
To circumvent this problem you can set a calculated default value that sets an empty string as the default value. This is an equal sign followed by two double quotes:

Now, when a new item using this field is presented to the user a blank item is selected. If the item is added without supplying a value an error is presented:

[...] Metadata, Default Values and Required Columns [...]
SharePoint Kaffeetasse 127 - Michael Greth [SharePoint MVP] - SharePointCommunity
September 25, 2009 at 10:25 am