What sheen should I use on the outside of a house?

Short answer

Use flat or satin (often called low luster) on siding, and satin or semi-gloss on trim, and semi-gloss or gloss on the front door and shutters. Flat hides uneven old siding. Satin sheds dirt and mildew better. Shinier finishes on trim make details stand out and are easier to wash.