The major problem with placing the lav off center (in the collar or in the collarbone area) of the subject's center line is the wildly varying audio levels as they gesticulate or turn their head. It's not just a level change but a tonal one as well. (The ear placement doesn't have this problem as the mic turns with their head). The other thing to watch for the collarbone area is bone conductance with the lav touching the actual collarbone/clavicle. Always try to get the mic centered on the subject and below the pit of the neck and above the solar plexus. Right in the middle of the sternum always sound best to me as you get a good mix of mouth sound and chest sound.