Headphones are an absolute must for handheld music games, unless you're a) alone or b) a jerk. You're almost required to have the sound on, unlike other portable games, and it would be quite rude to subject the other bus passengers (or other neighbors) to your racket. Guitar Hero: On Tour also puts your neighbors in danger of a serious jostling should your fake solos get too energetic, but there's not really an accessory that could help with that, aside from elbow pads.
GameStop has taken the extremely sensible step of bundling a pair of On Tour-branded headphones with copies of the game. If you're going to be buying the thing anyway, might as well get extra free stuff with it!
In other On Tour accessories news, the "Ruby Red" accessory pack is now pictured on GameStop's website.