Nate Silver runs down the numbers.  Let me also link to this post.  It cites a study (gated) of the 1932-2000 presidential elections by Richard Powell.  The upshot:

Generally, parties do not derive significant electoral benefits in states selected to host the national convention or those in which they control the governorship.

Emphasis mine.

[Cross-posted at The Monkey Cage]

John Sides

John Sides is an associate professor of political science at George Washington University.