As a poster above said, is it worth being ~20 minutes early every day to avoid being 10 minutes late some day?

If you are on time you are late.

http://www.wikihow.com/Be-Punctual

That's not relevant, sadly. It's just a matter of finding the point that will minimise wasted time (in both directions).

I don't think much of this thread is relevant. If somebody gives me a time to be somewhere, and it leads me to complex avenues of moral debate over the impact on my professional career and the opportunities in life for my children, for less than a kilosecond, then no.

Are they on time for work? Good. Are they late? Did they prevent or ameliorate that having an impact?

Will they execute what they say they will, when they say they will? We may have to wait on that.

I don't mind my employees being 10 minutes late one day and 10 minutes early the other. I understand that, with variances, you need to shoot for the mean.

no, people should be alotted a certain amount of variance for daily arrival times.

They only really need to be on time for meetings, where their lateness wastes N peoples times

Yep, obviously you should be a bit earlier for special occasions, but being 10 minutes late on a regular day is no big deal...