By the time you get Lich Bane you'll still have more AD than AP or right about the same, therefore you're getting more damage bonus out of Sheen. Best to upgrade to Archangel's Staff first then Lich Bane if you go that route.

Although best would probably be Tear of the Goddess > Sheen > Rabadon's Deathcap > Lich Bane > Archangel's Staff

Seen this build in action.
Quite nice dmg, especially when the enemy teams sees the sona as a support character.
The proc with power chord and lich bane is amazing as well :))

nice one tuna ^^