Wow what a thought. I too am of caucasian background 6" Slim and fit. I also started at Tiff's and other cauc establishments. I think some girls have their preferences but all in all I think it may have more to do with if you "click" with the girls and that some the of the authors ability to take some poetic license. After visiting some of these "outstanding" girls an AR has been written about I have come to the conclusion on some occasions that there may be another reason why the AR was produced or I just wasn't that girls type and didn't get the same level of service. On other occasions I have found a lot of honesty in the review.
I have met some girls that are open about preferring Again guys for one reason or another but I have also come across girls that tell me they prefer Aussie men.
So the way I see it (and I may be totally wrong) I don't think the service you get would be that much different to others unless you're an areshole to them, which I don't think you are, but there are some serious arseholes out there. It's more likely to be an exaggeration on the encounter.
Just my 2 cents worth.