Here's the thing. The Dollar kept roughly the same value until the 20th Century because the U.S. was on and off, mostly on, the Gold Standard. This included all the negative effects of the Gold standard, including long periods of deflation between major gold strikes, contributing to the boom and bust cycle, and generally hurting the poor at the expense of the rich. You should have read about in school, for instance, of the free silver movement, which grew out of populist dissent arising from precisely one of those periods of deflation.
Now that we have the ability to maintain a fiat currency and still retain popular confidence in the monetary system, there's absolutely no reason not to maintain a small positive inflation rate which has a whole series of benefits.