What names can you not give a character without creating a distracting reference?
Hitler and Stalin, obviously. You might get away with Churchill, probably with Patton, certainly with Montgomery, probably with Rommel. Not with Goebbels or Himmler. Goering maybe, if you spelled it Gehring. Eichmann perhaps -- it's not that uncommon a name.
Not with Roosevelt, although in one of those Ogden Nash prose poems that so influence Russel Edson, though no one but me remembers them and makes that claim -- anyway, in this Nash poem there's a butler named Roosevelt. The protagonist asks him if he's any relation to the Roosevelt, and he replies that all Roosevelts are the Roosevelt. Not with Vanderbilt or Rockefeller, possibly with Astor.
Moving back to presidents, you could certainly use Kennedy or Johnson or Carter, maybe not Nixon, although it's nearly as common a name. Ford -- if his name raised irrelevant associations, they'd be to the car, not the president. Reagan and Bush and Clinton -- maybe not now, but in a few years they'll be OK.
McVeigh, no way. Oswald...it's a common enough name that maybe you could, but of you gave the character a first and middle name, you'd be sunk. And certainly, you could not give a character the first and middle names of Lee Harvey.
I can't think of any 20th century poet whose name you couldn't use, either because they're too common -- Williams, Stevens, even Pound or Eliot (although if you used them both you'd be sunk) or because they're too obscure. Maybe Plath would be a problem.
The only 19th Century American writer you'd have trouble with is Longfellow -- at least the only one who occurs to me. Twain isn't really a name. Poe...you could slip that one by.