You didn't mention your OS, but nonetheless......
If you have nothing running on your computer there should never be anything in the "temp" folders. But files do get put there and not removed by programs, also installation routines often drop things in the temp folder and don't clean them up. Viruses also love the temp folder, it is easy to find.
I have seen people save things in the temp folder because the program requests to be saved there and they don't know any better.
But, assuming good computer use practices, yes it should be safe to delete anything in the temp folders (I do it all the time). Go to your temp folder and click the first file, hold down shift, click the last file, release the shift key, (or use <ctrl> + A). All files are now selected. Hit delete. You may get one or more messages about deleting read only or exe files, just let it delete them. If a file is unable to be deleted chances are that it is in use and will be removed when the program using it closes.
