That "rule" is utterly wrong. I've heard it before, often to justify why an otherwise respectable store doesn't bother stocking size 30 belts, and never understood why people continue to repeat it. Your belt size should be the same as your waist size- you're supposed to use the middle hole, with the others to provide some give in either direction. Belts are measured to the middle hole- if you buckle a 32" belt in the middle hole, it will form a loop 32 inches in circumference.
Now, with vanity sizing and the expectation that you'll only get fatter instead of any thinner, getting a larger belt might be justified in some cases, but should hardly be considered the rule. I wear a size 29 or 30 waist depending on the pant and the day, and wear exclusively size 30 belts.
This can not go unchallenged. I wear size 34 pants -- all my belts are 36. Before I embraced the Ben & Jerry's subculture, I wore size 32 pants and all the belts were 34. Go figure. OH! OH! an epiphany ---The pants waist band, collectively = the 2". That must be it, since I recall when I lazing out at the nudist resort, I wore the same size belt as my waist measurement.