Is it obligatory to recite the opening supplication (Du'a al-Istiftah) when joining a congregational prayer that has already begun?

In a silent prayer, if someone joins late while the Imam is standing, if he knows the Imam will not bow before he completes Al-Fatiha, he should recite the opening supplication (Du'a al-Istiftah). If he thinks the Imam will bow before that, he must only recite Al-Fatiha. As for loud prayers, it is obligatory for the follower to listen attentively to the Imam and not recite anything other than Al-Fatiha. The opening supplication is a Sunnah, not an obligation.