Recently, I have strugling with strange issue in which my C# project constantly builds. This might not look like a big issue, but multiply this by 20 and you will end up wasting considerable amount of time. I have looked for any ideas, being logical or not - from tracking if any file is modified to whether the xaml compiler make some hacky things.
What I found out is that the problem was located in the Content and Resource file types - if by any reason its action is Copy Always it will trigger full build, even they does not affect the other output (does not sound very practical). Nevertelless, setting in to Copy if newer resolves the issue.
P.S. If the problem persists you might what to delete the *.suo files as well (Thanks Zhoro for the tip :)).