This is especially true for first impressions. If you don't want to leave a bad impression, make sure to be on time for any appointments.
Although there might be cultural differences based on where you live, remember to treat others how you want to be treated. Unless you enjoy pointlessly waiting on others, don't make others wait.
If you're running late, it's not much of an issue as long as you give a heads up. Even if it's just a few minutes, give a heads up.