I am not an Adobe Lightroom user (though one colleague and one former student may sway me from my Photo Mechanic/Aperture combo), so I don’t have a lot of experience here to back this up … but Jeffrey Friendl has posted an excellent tutorial on JPEG compression quality as it relates to Lightroom exports. It has some fantastic interactive sliders that show you how various quality settings will affect your final image.
Even if you don’t use Lightroom, it’s a clean way of learning how JPEGs work. Worth the time.
(Picked up from Daring Fireball.)