There are a couple of things to consider with negative camber.
It will help your cornering ability at high speeds, however it result in increased tire wear at city driving speeds. The nice thing about adjustable camber plates is that you can easily change from -3 deg to -1.5 degrees without too much difficulty.
It will mostly help tire wear on the track, as the outside edge (typically the left front is the primary concern) will get the most use. If you can spread the wear across the tire, they'll generally last longer.
Too much negative camber is a bad thing on the streets; one, your inside edge of each tire will wear out quickly. Two, your car will tend to hunt and dart through each ridge in the road; basically, the ride will really suck.
And yes, negative camber should be on both the front and rear wheels.
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