I'm with the others, book as early as you can. It's highly unlikely that any sort of sale airfares would be released close to Christmas.
Also there is no problem with booking US trips direct on an airline website without a TA with a few caveats. First remember that unless the airline is offering SYD-LAX-LAS for example where the airlines will get you all the way to LAS in the event of a problem. If you book two tickets, a SYD-LAX, and then booking an LAX-LAS ticket separately (say on a much cheaper airline) could mean that you end up stuck in LAX if you miss your connection, the airlines have no responsibility to help.
I've booked my last couple of US trips directly via the web without a TA and didn't have a problem. In fact the one thing I left up to the TA to book on my last trip, they managed to screw up (it was flight centre, you don't need to read very much here to get the general feeling about them). So a TA is no guarantee that things will run smoothly.