Speaking as both a player of bards and as a player who usually enjoys other people's bards, I do think that picking the music carefully is key.
Whether you choose to go the more descriptive route, the C&P-the-lyrics route, or some combination of the two, make sure the music fits the setting.
Now by that, I don't mean that you shouldn't play darker tunes in the CRT or whatever. What I mean is, make sure the music has a flavor appropriate to the setting. Traditional folk music isn't all sweetness and light. Ever really listen to "Twa Corbies"? Yeesh, how much more depressing can you get than a couple of ravens discussing picking apart the corpse of a lone knight slain in his travels?
For that matter, modern music isn't always inapproriate, as long as the style works with the setting. My own bards (admittedly sunshine-and-daisies types) have performed songs by Lorena McKennit, Heather Alexander, Mercedes Lackey, and an SCA-bard that I had the privilege of meeting some years back when I spent a couple of years in Atlantia. Heather Alexander, at least, does have some slightly darker and more agressive tunes... "Courage Knows No Bounds" is depressing, and "March of Cambreadth" is definitely agressive, with its tag line of, "How many of them can we make die?!?"
The one thing that really bothers me is when people take a thoroughly modern tune in a style that isn't even close to appropriate for the setting, and just tweak a word or two. I happen to love Iron Maiden, but I did not love someone's rendition of "The Trooper" as performed in the CRT. Sure, the person performing it used fireballs and arrows in place of cannonballs and bullets, but I really don't think it worked overall, because the song is too recognizably real-world modern.