Can I set a field using a string (example, setting the field Subsidiary to "Ace Hardware") or do I have to use the internal ID of the value I want to set it to?

Typically, it's "best practice" to set the list ID value, as this will ensure any future modifications to the text (UI) don't crash your script. That said, the following may be possible:

objRecord.setText({ fieldId: 'subsidiary', text: 'Ace Hardware', ignoreFieldChange: true });