The current interface for defining default values of Multiple Choice fields does not allow several operations, including:
* making all values selected by default (in case of multi-selection field)
* making no value selected by default after you've selected one
I think the Knack interface should use checkboxes instead of radio buttons for multi-selection fields, and it should add an extra radio button for "None selected" for single-selection fields.
I'm currently using JavaScript to perform these default selections.