NKP, I do both.
-I use the photographs of REAL cars to get inspiration, or in some painstaking examples, copy it exactly as I see it/
-I Take photos of my own work to look at it in a whole different way, to catch mistakes, and to have it critiqued by others on how I could improve it.
-Finally, after things have been all revised, I take an "offical portrait" of my work to keep for posterity.
If you're referring to the first post, many of the skilled people who weather their models go out into the field and take photos of an object similar or identical to their model, and using those photographs, create a miniature model that looks just like the real life object. If they cannot find a real-life object that looks like their model, it's an obvious next-step to look in books, magazines, or online for that illusive, or rare object.
(ex. You obviously cannot find a actual, weathered NYC J-class Hudson, because they were ALL scrapped in the 1950's but there's a good amount of color film and photographs to create a plausible model of one, although it's impossible to see it for yourself today in 2008.)
Some people take shortcuts and ignore this step, and get weathering that seems like "something's a bit off" like having rust in strange places, or putting graffiti in places on a car that a tagger normally wouln't or couldn't reach easily. Most people who do this are prone to exxagerating their weathering or make it too even, or bringing this topic full-circle to Hoppercarmaker's original comment:
"The sellers are just spray painting spots and putting what looks like dirty water on them and calling it weatherd."