I am able to invoke the check spelling box by:
Please see the below screen shot for the exact requirement.
This checkbox is related to "Add" button, so it manage adding words to chosen dictionary in case sensitive or unsensitive mode. Is it a workflow you were looking for?
So this dialog is complex since spelling and adding word to a dict - both methods - are connected in one place.
As far as I know, there is no addWords preferences.
From the other hand - this dialog looks like "user interaction" by default, so some options can lay outside of script area.
ID UI seems to remember this button "state", but to the end of ID session only.
Thats why is hard to believe this option is connected to preferences.
Moreover, checkboxes from dialogs "Dictionary..." and "Check Spelling..." are connected, so anyone ON/OFF involves another to the same state.