Crossing one self is a silent prayer and should be accompanied by a silent prayer, even if it is an assent to what the priest is saying or doing. I bow as the Beautiful Gates are opened, but I could cross myself and say my own prayer if I felt so motivated.
Typically, one would cross one self at the mention of the Holy Trinity and the Theotokos; but what of the words of Institution? How about as an assent to the petitions of the Divine Services?
Orthodox worship is not so structured that we cannot be moved by our own personal feelings to cross ourselves, or venerate an Icon. The church does not teach that you must or shouldn't cross yourself at any point in the Divine Services, except perhaps for the practice of not crossing one self during the reading of the Six Psalms, due to respect for the traditional practice not to move during their recitation.