All files *.mode*, *.pbxuserand *.perspective*can be safely dropped without affecting the organization of the project. These files are used by Xcode to track which documents are open, which windows are displayed, what are their sizes, and how your project looks.
I ignore everything except the file project.pbxprojwhen managing projects in version control. Since I'm using Subversion, I have the following line in ~ / .subversion / config:
global-ignores = *.o *.lo *.la .*~ ._* .DS_Store *.pbxuser *.mode* *.perspective*