If client data is never added for a particular QID, the cell is left alone. For example, if there is a rating spreadsheet with a 'deductible' cell that has a $500 value in it, and that value is mapped to a question - but then left unanswered (null value in the answer field), the system will use $500 as the default deductible in any calculations.
You are able to make answering a question mandatory, which would force the end-user to put a value in for a field.