According to Kallistos Ware Orthodox are free to believe it.
The Orthodox Church calls Mary ‘All-Holy;’ it calls her ‘immaculate’ or ‘spotless’ (in Greek, achrantos); and all Orthodox are agreed in believing that Our Lady was free from actual sin. But was she also free from original sin? In other words, does Orthodoxy agree with the Roman Catholic doctrine of the Immaculate Conception, proclaimed as a dogma by Pope Pius the Ninth in 1854, according to which Mary, from the moment she was conceived by her mother Saint Anne, was by God’s special decree delivered from ‘all stain of original sin?’
The Orthodox Church has never in fact made any formal and definitive pronouncement on the matter. In the past individual Orthodox have made statements which, if not definitely affirming the doctrine of the Immaculate Conception, at any rate approach close to it; but since 1854 the great majority of Orthodox have rejected the doctrine, for several reasons. They feel it to be unnecessary; they feel that, at any rate as defined by the Roman Catholic Church, it implies a false understanding of original sin; they suspect the doctrine because it seems to separate Mary from the rest of the descendants of Adam, putting her in a completely different class from all the other righteous men and women of the Old Testament.
From the Orthodox point of view, however, the whole question belongs to the realm of theological opinion; and if an individual Orthodox today felt impelled to believe in the Immaculate Conception, he could not be termed a heretic for so doing.
(empahsis mine)