Technically, I believe that you're supposed to use the site of the country where you are resident. In practice, I (as a US resident) have booked reservations in the US using the UK and German sites many times and had zero problems.
One thing I'd mention - if you're not a US resident (and hence don't have US car insurance), and you book a US rental on (for example) the German site, it'll automatically include the necessary insurance. It won't do so if you book on the US site.