For many years I attended customer sites all over the UK and even into Euroland. Making sure that I attended the customer site at the time we had agreed was, for me one of the most important things I could do.
If I said I would be there at 09:00am then that is the time I walked through the door. Not 08:45am because being early means the customer may not be ready for me. Not 09:15am because that is not professional and it means I start the appointment with an apology which never looks good.
But more that anything, not attending at the agreed time is very disrespectful.