I've always liked this article on the matter, which links to additional reading on the topic of assassinations. Although this isn't exactly what you are asking for. Also, its about world wide assassinations, not just US. The gist of it is that:
assassination of autocrats tend to lead to a more democratic system
assassination in democracies do not have as much effect on the governing system
assassinations tend to increase small scale conflicts, but actually hasten the end of large scale conflicts