Based on the fact that half the time one can barely understand what they're singing forwards [sic!], it seems unlikely one would understand anything at all when it is sung or spoken backwards. And in any event, if a faithful Witness heard something that objectionable (even subconsciously), wouldn't that neccessarily result in an unwillingness to continue listening to the song?
-- bennyk (of the "yeah, my little sister tossed my Led Zeppelin albums" class)