CRM 2011 Account Initialize Default Values

The account entity has 3 custom fields that are Business Required.

When user creates an account, JavaScript in the form OnLoad, initializes these 3 fields with default values.
When instead the account is created by a process such as "Qualifying Lead", those 3 fields are saved with no value.

What would be the best way to make sure that no matter how the account record is created, these 3 fields will always be populated with their default value:

- workflow (what type)?
- plugin (what type)?
- other?

Thanks and best regards.

Frank