you can get right up on the cars and still get them all in the frame.
The problem is, at car shows, you've got
lotsa people milling around, walkin in front of you and the cars...but, with a wide angle lens you can get right up to the 'ropes' and people won't be able to walk in front of you and stand there in your way/view!
I use my 17-70mm Sigma at car shows (or, in past, the 18-55 kit lens). I shoot the NAIAS held in Detroit every year, and if you click on my links (below) you can view the 2008 show (Flickr site).
Even if the cars aren't 'roped' off, the closer you can get to your subject/car, the better chance you have of people getting in your way.
As for WB, I use Auto WB, generally...or, you can set up your WB with the 'preset WB' if all the lighting is the same...or, just shoot in RAW and you can set the WB in post-processing.
My photog bud that usually goes to the shows with me uses a Canon 30D with a 12-22mm lens (I think that's the focal length).
We also do a lotta car shows outside, and the lighting is pretty harsh on sunny days...but, you can always 'fix' your exposures in post-processing when the WB/exposure is off.
I'm lusting for a 10-20 or 12-24 lens, but right now, $$$ is problematic ;-)
Oh...one more thing, regarding flash...I sometimes use the onboard flash at the shows if/when the lighting in a particular area of the show is weak. I've never had a problem using the flash at any show/s. But, it's not generally required since the lighting is usually decent at the bigger shows. The problem with using flash though is, it oftens puts a nice big bright spot on your car and blowing out your subject/highlights.
You may have to kick up your ISO a bit if you don't have a fast lens in order to get shutter speed high enough to avoid blurred shots.
You will probably mess up a few (or, alotta

shots since you're new to shooting in that type of environment, but with a bit of practice you will get the hang of it and get some decent results.
Good Luck...
--
Jim K...just outside Detroit, MI
DeeEighty; DeeFifty; CeeTwentyOneHundred; EffZeeFiveK
http://www.pbase.com/jkorsog
http://motorcityjim.fotopic.net/
http://www.flickr.com/photos/motor_city_jim/