Which rockers love their fans most? Once it was the Grateful Dead; now it's those metalmeisters Metallica. Throughout its North American tour, which began two weeks ago, the band will allow 50 to 100 fans at each show into a prime section behind the mixing board to make personal audio- and videotapes of its three-hour concert. ''It's the ultimate souvenir to take the show home after you've lived through it,'' says drummer Lars Ulrich. But aren't most rockers / fearful of bootlegging? ''It's not really a big deal,'' Ulrich maintains. ''Who am I to sit on my high horse and say, 'I'm a rock star and you can't do this because it takes away from our record sales'? We don't have a problem with record sales.'' Ulrich, who claims to have the world's biggest Deep Purple and Black Sabbath bootleg video collection, says he actually appreciates enthusiastic fanatics who've got ''like, 247 different audiotapes and 49 videotapes of our shows.'' Listening, Axl Rose?