While the Orthodox Church certainly considers Theotokos to be immaculate in every others way, by her own choice to obey God, she is as human and capable of sinning as any one of us.
To the Ortodox mindset, God's intervention in the moment of her conception by her parents would separate her from us and make her sainthood not the highest feat of faithful cooperation with God's will, but simply an act of God's will, and not a result of her theosis.
As a "mere" human, she gives hope to all, and serves as a model for all to follow and venerate. In the Orhtodox Church she is the "highest" saint in terms of honor. But the very fact that she was deemed worthy of her awesome task, and that the Word used her flesh to become Man, speaks volumes of her purity, for we all believe, I hope, that the flesh God used for Incarnation was not defiled or anything short of immaculate. And the mere fact that she gave Birth to the Savior of or souls makes even any discussion of her "impurity" meaningless. She has indebted all of us with her love.
The fact that both sides of the Church, Greek and Latin never questioned her immaculate life is a testament that this was understood to be that way from the beginning. Both sides of the Apostolic Church believe that she was assumed bodily into heaven on the third day after her death, although there are some Catholics who deny that she ever died. Some Orthodox even point to the fact that she may have sinned (the incident at the wedding party when Christ turned water into wine), but these are private opinions and not doctrinal statements.
You're right, Kosta, I wasn't clear on your take. Thanks for the info. And, just to be sure, you are saying that Mary could have sinned, but all through her life she chose not to, so she was sinless in terms of "performance"?