I just shifted my residence and coincidentally one of my direct reports live nearby, so in order to save time I go to office with him. Likewise, upon return one of my other subordinate drops me half way towards my home this saves me energy and time as I live very far away from office.
From an objective point of view, I do not think it has any impact as I at the workplace I conduct myself very professionally with them and this picking up and dropping at home does not in any way impair objectivity and/or does not in any way discriminate them with returning the favors.
From HR stand point is it okay to get such small favors from subordinates?