The answer:
Absolutely not. Since 1971, postal rates have increased more slowly than the actual inflation rate, as measured by the U.S. Consumer Price Index. So, despite the numerous rate hikes over the last 36 years, stamps have actually been getting cheaper. The 20-cent stamp from 1981, for instance, would be equivalent to 45 cents in today's dollars—which makes today's rate 10 percent cheaper than it was 26 years ago. Should this historical pattern hold, you'd be paying more for today's forever stamps than you would for any stamp in the future, no matter how high the rate goes.(Via Cynical-C.)