If you ring an airline, chances are while you are waiting on hold for anything up to 45 minutes that one of the hold messages will say “You do not have to reconfirm your flight”.
They may say it, but is that really the case? Very often yes – but not always, and especially not in the USA.
One of the times to definitely reconfirm your flights is when there is a change of time for instance to daylight savings time or back from that, as that will adjust arrival or departure times by up to an hour. You do not want to arrive at an airport for an international flight with only an hour to spare to get through all the formalities, so if you are departing around about the changeover time always check there has not been a time change.
The other time to ALWAYS reconfirm your flights is travelling within the United States.
USA airlines routinely overbook their planes and if you have not reconfirmed the flight you are booked on chances are you will be bumped off and offered a later or different flight. This is NOT useful if you have a connecting flight! And it is not useful if you have pre-booked a hire car, or if someone is meeting you at the other end to transfer you to your hotel.
A great rule of thumb is to do two things. One – do your seat allocation before you depart your home city if the airline allows it. The airline is less likely to bump you off if you have gone to the trouble of getting a seat pre-booked. Two – as soon as you arrive in your city, reconfirm your onward flight BEFORE YOU LEAVE THE AIRPORT. That way the airlines knows you want to be on your booked flight and not to be offered $50 to take a later flight.
And while travelling in the USA be warned – most domestic airlines there do not include checked luggage in their fare structure unless it is part of an international ticket. You can expect to pay $US20-35 per bag for checked luggage. You can sometimes add luggage online more cheaply than at the airport. This also explains why on most domestic US flights travellers have so much huge hand luggage – they are taking everything they need in what passes for hand luggage. It does not leave very much room for anyone else so boarding early is recommended.