From my experience, the tabernacle is always on the altar. But I've been told that some of the churches have no tabernacle, as all of the Body and Blood are consumed that day, and are made afresh every liturgy. Now I've never seen this in an Orthodox Church, but in some Catholic and Anglican parishes, the sacrament is moved to the side. The purpose if which I assume is to free up movement. Bur ti make one have to , if you will excuse the phrase, "Find the Jesus". It is because the alter has been changed from the centerpoint back to foreward.