My rule of thumb is:
If a user is presented with the opportunity to fill in a value for the field, then it should be an empty string or a default value (0 for numeric),
if the user was never presented with the option to fill in a value for that field, then save a NULL


>I understand NULLS have a special purpose... but I think I'm starting to abuse it. For example, if we have a text comments area that saves to a varchar field, would it be proper to use NULL if the user left the text area blank, or would it be proper to use a blank string?