I am now going to go against what everyone else said

I recommend booking your international from Perth book the internal flights in the US separate but not from Perth my reasons for this are on our way home our plane was 10 mins late arriving in Brisbane Customs took there sweet time and we missed our connecting flight back to Perth, Qantas put us on the next flights and gave us meal vouchers no extra cost to us but other people had them booked separate and it was there problem i for one would not want to be paying the extra money to fly home that day. You also get international luggage all the way back to Perth which is 2 suitcases per person.
The first year we went we booked our flights separate as we had something on in Brisbane on the way home but we flew over to Sydney the day before our flight to the USA just to make sure that we didn't miss our flight so if you add in the cost of hotels we saved nothing.
Make sure you also look at the Dallas route as often its the same cost as to LA and also when looking at other airlines make sure you compare the travel time as some of them especially through Asia arrive the next day not the same day.
And Welcome to the boards