Is it obligatory to cover the face during prayer when men are present or expected to pass by, or is there no harm in uncovering it?

A woman's entire body is considered 'awrah (private parts that must be covered) during prayer, and she is obligated to cover her entire body except for her face. If she is alone or with her maharim (unmarriageable kin), she may uncover her face. However, if strange men are present, she must cover her face, both during prayer and otherwise.