Browser: Meh. iTunes? Give that one a little bit of thought (but don't lose sleep over it).
Files that are open as writable while the backup is running aren't going going to get backed up in a useful state, if at all. For iTunes that means your library (the database part) may not be backed up properly (but the music and videos themselves should be okay).
For a browser, downloads in progress and cache junk would be the victims. You probably don't care about 'em.