I have nothing to add to what Sean Wilentz said about Ulysses S. Grant, other than to say that it's worth following it up with what Matt Yglesias said about Grant, race, and conservatives. The postwar portion of the 19th century is definitely not my specialty, so I'll defer to those who know more about it. From what I do know, they sound right to me.
That said, I can talk a little about which presidents we honor too much, and which we honor too little. On the former, James Madison deserves far more celebration than he gets. I mean, Washington gets his obelisk, and Jefferson and Lincoln get their temples, but the official Madison memorial -- and I didn't know the "official" part until just now -- is a building that....well, I actually am in what I believe is a minority who do not consider it a complete monstrosity. C'mon -- Jefferson not only gets his memorial, and the nickel, and the $2 bill, and Rushmore, but when they finally decided to name something after Madison, they only did it along with naming the much nicer and far more beloved building next store after Jefferson. Not to mention that Congress went and named their bill search engine thingy after Jefferson, despite the fact that he never actually served in (the currently organized) Congress. Madison was pictured, by the way, on the no-longer-in-circulation $5000 bill, which puts him right there with Grover Cleveland ($1000) and William McKinley ($500).
Move up to the 20th century, and stick to currency and coins, and the president who is conspicuous with his absence is not Ronald Reagan, but Harry S. Truman. Truman is generally rated quite highly by most historians and political scientists, and on the specific question of winning the Cold War, Truman (in my opinion at least) towers over the rest of the lot, who essentially just carried out the strategy that Truman put in place. If I had to rank presidents just on managing the Cold War, which as Wilentz notes is Reagan's best argument, I'd have to say it would be Truman, then Ike, then Bush, and then let Reagan fight it out with the rest of them.
I'd say Truman, but others might argue that Teddy Roosevelt deserves more official recognition. I figure -- the guy has Rushmore, and that's enough. But others would disagree.
As far as overly honored, that's easy -- other than the oddball and purely symbolic Sacagawea choice, the one figure who is currently honored on American currency and coin despite little historical importance is John F. Kennedy. I suppose that it's unlikely that Kennedy will be bumped until the last baby boomer dies, but if it were up to me I'd probably push him aside for Truman, and let poor Madison have the $2 bill. Jefferson will survive.