Is straightening the rows before the prayer obligatory or recommended?

Shaykh Ṣāliḥ ibn Fawzān al-Fawzān:

It is obligatory to straighten the rows. The Messenger ﷺ would not begin the prayer until looking at the rows and ordering that the shoulders and ankles be aligned in their correct positions. Thereon after if the rows had been correctly straightened he ﷺ would begin the prayer.