Has anyone out there prebooked tours before travelling to the US. Is there any benefit, is it cheaper to do it whilst there or be super organised do it on-line from Australia. Any comments would be appreciated.

Where in the U.S. are you going? The answer to your question depends on where you are going and what kind of tours you are planning.

San Francisco, Las Vegas and Los Angeles. Talking about day tours, such as Grand Canyon, Alctraz etc.

I'd say book them when you get here.
Anyone else?

Wait until you get here to book them. Who knows what the weather will be on any particular day - too hot or cold or windy or rainy - or what you'll feel like doing.

I would book the tours when you get to each city and see what your mood is.

We sometimes take a tour on the first day we are in a city - gives us an overview, a bit of history and some ideas about what we want to return to.

Have never had any problem booking when we arrived in the city, even on same day as tour. There are usually several companies that run nearly identical tours and we often pick the one which will pick us up right at or near our hotel. Sometimes there are even discount coupons that can be found in booklets, brochures that they have at hotel.