An argument is non-valid if, but only if, there is a row in the truth table for the argument in which the premises are all true AND the conclusion is false.  Accordingly, those rows in which the conclusion is true could not possibly demonstrate non-validity, those rows could not constitute invalidating rows.  So, we are not interested in such rows when testing for validity.  what does this tell you about an argument whose conclusion is a tautology?  Click here for the answer.