When you export or email files, you are able to specify output options that effect just generating the exported files.  To do this, open the Email or Export screens from the Share Menu and then locate the wrench in the lower right corner.  Tap on this to open the Output Options Menu and enable the Lyrics Only option by tapping on it to place a checkmark.  Now when you export or email the files, OnSong will only display lyrics without effecting the songs in the Song Viewer.