There is much talk about how Kennedy would have been regarded if he had lived, and how Reconstruction might have been different under Lincoln, but I rarely hear much about Garfield or McKinley.
So, did their reputations change much in the aftermath of their deaths, or were Lincoln and Kennedy exceptions?
Garfield was already a beloved national hero due to his service in the American Civil War. He was only president for a few months before his assassination.
Source: Destiny of the Republic
That being said, I believe his reputation has actually suffered due to his assassination. He wasn't in office long enough to create any sweeping legislative changes like Kennedy or Lincoln, so there is less memory of his achievements. Any that were made have certainly not been as long lasting.
I made short write up of it here as well. http://www.reddit.com/r/AskHistorians/comments/2n6r0t/why_are_the_assassinations_of_jfk_and_lincoln/cmay88r?context=3