I don’t think hoarding would be all that much of a problem, honestly. Sure, some people would, but most people wouldn’t bother, particularly since rate increases are so small. You’d use what you have, go out and get some more, use that. Before a rate hike there might be a little run on first class stamps, but I think they’d make up for it with the savings of just having “first class stamps”.
Yeah, I can’t imagine someone going out and getting a few hundred thousand stamps, then selling them after the rate increase for a penny or two below the new value, causing issues for the post office.
Sarcasm duly noted. And quite apt. 
Yeah, but in the meantime, the Post Office has been paid beforehand for a few hundred thousand letters that it didn’t have to move around, so they can leave that money somewhere collecting interest, right? I think it might break about even, but I don’t really want to do the math right now.