This seems like an obvious statement. But too many real estate agents underestimate traffic or spend too long on a phone call prior, or for a host of other reasons, arrive late for the appointment. Your seller prospect can only be assuming that it's probable that your marketing and other listing functions might also be late in delivery.
Early is not better either. In trying to make their home as presentable as possible, the seller(s) may be bustling around at the last minute. Don't arrive early and add that pressure to the meeting process.
In short, even if you have to drive around the neighborhood for ten minutes, arrive at the appointed time.