This is not something that is supported in FormsCentral for online forms. If you collect data via the PDF form then you could use Acrobat to set the default values for each fields before you distribute you PDF.
I'm not sure where the feature request for this has gone, appears to have disappeared. Anyways, +1
I believe being able to select default values for fields, regardless of their type, would be an asset to this application and to Adobe itself. I am currently working on a form that this would come in handy, hoping for an update soon, hint hint
edit: If you are not currently investigating this, a simpler solution would to allow logic to apply on null values. This wont work for all cases, but it will work for mine.
Thanks for your input. This is not something that we are currently working on but will keep note of it.