The voucher can be used for the full price printed on the ticket, including tax. The voucher isn't good for airport departure taxes paid in cash on day of departure- but relatively few airports charge cash fees.

I've heard that international taxes are reduced/waived if a ticket is paid with a voucher, and once with AA I succeeded in ticketing a US-Mexico reservation with only like $20 of a total $60 in tax assessed. Does anyone know the official rule or policy? Agents with NW and other airlines think I'm nuts when I mention it!