I think you'll want to use Field Validation to ensure that a value is entered prior to saving.
A combination of "Empty List Selection String" and "Allow Empty" can be used to set the initial value to blank on new records (or records that have a null value in the db).
Have a look here to see if this would help:http://dragonrad.com/twiki/bin/view/Help/FieldValidation
Also discussed here:http://dragonrad.com/smf/index.php?topic=34.0
I'll update our documentation a little for the "Empty List Selection String" and "Allow Empty". It should be covered here:http://dragonrad.com/twiki/bin/view/Help/OtherFields#List_field