[Don W]

A good rule of thumb is any number that you have to add letters to should be written out.

Awesome rule. What about eras/dates EG 1980's ?

Is Nineteen Eighties clearer?

Just being a smarty-pants.

[Harmless Teddy Bear]

Actually you should eliminate the Nineteen all together. So it would just be "the Eighties".

You could also give a date range (IE from January 20, 1981 - January 20, 1989 was known as the Reagan Era)

But as a general rule of thumb you should not mix numbers and letters.

As with any general rule there might be exceptions.