In my opinion, there is nothing ruder than a person who is habitually late. Even if you’re only one minute late, you’re still late. It’s selfish and disrespectful to the person or persons stuck waiting for you. What about their time? Isn’t it as valuable as yours? Yes, I understand things can go wrong, but let this be the exception not the rule. If you know it takes you an hour to get ready don’t start thirty minutes before you need to leave. A wise person would start an hour and a half earlier, giving themselves extra time for the incidentals-of-life that crop up.
I don’t care if this was the way you were raised. It’s no excuse. We all have bad habits. Anything can be overcome with a little self-control. Do you really want to be the kind of person that no one trusts, or the butt of every joke at a group gathering? “Oh the so-and-so family’s late again. Surprise, surprise! Thank goodness we didn’t put them in charge.”
Do yourself a favor, have a little class, and make sure you always show up on time. Really, a few minutes early would probably be best. Everyone, not just your employers and college professors, will like you a whole lot better.