The SourceSafe action will fail if ss.exe returns an exitcode other than 0, and since ss.exe returns an exitcode of 1 on a diff if 1 or more files are different [1], that will cause the step to fail. We'll see about adding an option to the SourceSafe action to treat an exitcode of 1 as success.
[1]
http://msdn.microsoft.com/library/de..._exitcodes.asp