xdg-open is a desktop-independent tool for configuring the default applications of a user. Many applications invoke the Template:Codeline command internally.
# This will make Thunar the default file-browser, i.e. the default application for opening folders. xdg-mime default Thunar.desktop inode/directory
The config file
[Default Applications] mimetype=desktopfile1;desktopfile2...
[Default Applications] text/html=firefox.desktop inode/directory=kde4-dolphin.desktop;Thunar.desktop;nautilus.desktop
It seems that it is perfectly save to edit that file.
If you want to set your default applications system-wide, copy this file to Template:Filename
export BROWSER=chromium # this will make xdg-open use chromium for any kind of URLs
export DE="xfce" # This will make xdg-open use the more user-friendly exo-open, because it assumes it is inside Xfce. Use exo-preferred-applications for configuring.