When I have had job interviews, I always drive the route the day before and then I plan to arrive 30 minutes early (planning for traffic situations that I can not control) and then if I arrive at the building 30 minutes early, I sit in my car and wait - walk in 10 minutes before the interview time. Always 10 minutes. 10 minutes does a few things. It shows that you are READY for the interview at it's start time, not fumbling around with things at the start time. It also shows the interviewer some respect and it gives you a chance to observe people walking around the office. Does everyone look miserable or content? Do people interact well with each other? Does anyone seem rushed?
When I did my first interview for the radio station in Ann Arbor back in 2004, when I was in the lobby waiting, one staffer walked by and said "are you sure you want this?", walked into the PDs office, talked to him a little bit and walked out laughing and as soon as he looked at me he stopped laughing, got this dour look on this face and as he walked by me, shook his head and said "you poor bastard"......I knew I would fit in well there.