Do I need to mail a voucher if I paid my taxes online?

No. If you paid your IRS or state taxes online through TurboTax, there's no need to mail a payment voucher. And if you paid (or will pay) your taxes but not through TurboTax, ignore the payment voucher printing instructions if you have to select Check Payment to get past the payment screen.

Sending in a voucher without an accompanying check or money order is a waste of paper and postage and generates unnecessary work for the government.