As we have noted elsewhere, when the Precious Cross is in the centre of the temple (e.g. for the Feast of the Exaltation and its afterfeast) we always prostrate ourselves before it, even if it be a Sunday (an exception to the otherwise strict injunction against prostrations on Sundays).
The one occasion on which we categorically do not prostrate ourselves before the Cross is on Great Friday of Holy Week, when the Cross (generally a very large, upright wooden one) is in the centre of the temple following the service of the Twelve Passion Gospels. On this sole occasion of the whole year, we do not make a prostration before it, out of memory of the event recorded in the Gospel for those days: that the soldiers mockingly bowed/prostrated before Christ. On this lone occasion we only make the sign of the cross over ourselves before it, and kiss the Cross.
(As a Holy Week guide: once we make the last prostration of the final Prayer of St Ephrem during Great Wednesday’s Liturgy of the Presanctified Gifts, we make no more prostrations at all until Pentecost, except for before the Holy Shroud.)