May have been covered, but I figure good to bring up again.
I've been using the same gapping tool (crappy autozone) for a few years and never seemed to have a problem in other cars. I went to change my plugs again recently and noticed that like previously my new plugs were under-gapped out of the box. Well, being a man of science I decided to double check that with a second tool that was older yes, but actually worth something. The difference? The factory gap was spot on, and my car feels like it has 25 lb-ft of torque more coming off the line.
Moral? I dunno. Don't use cheap-o tools? Check twice, gap once? Whatever :rolleyes: I think I'm getting slightly better fuel mileage now too.

I just changed my 04 WRX plugs at 58000 Saturday with new ones from the dealer, and 2 of them were slightly small with my feeler gauges. Like maybe 0.002 or so. I gapped them all the same, threw them in and the difference in driveability is unreal! Dont use those round gauges, you will have an angled gap, and I would bet the angle wont be the same on all of the plugs. Quality tools and taking your time FTW. :)

See, my 1969 Camaro with Mallory HEI didn't care about the gap quite as much...