The outer walls of a rack unit are always a bit more than 19" because the rack ears of audio gear are never perfect. You don't want metal ears sticking out the side of your desk. Personally I tend to give at least a 1/4" extra (total of both sides) to the walls of a rack when I'm building from scratch.
The inside has to be at least 18". Most audio gear is only 17" wide, with a 1" ear on either side. But many items are up to 18" wide, with only a 1/2" ear on each side.
For example, if you're using 3/4" wood, I would recommend putting 18-1/8" between the outer walls. Then your rack ears will stop 1/4" from each outer edge of the rack. As long as you're not using plywood for the racks, this will look nice.
The official height of 1 RU is 1.75" but again this is never perfect. For a 1U rack I would give at least 1/8" extra space; for a 6U rack I would give at least 1/4" extra space; and so on, adding a little bit for every few units.
If you have hot gear (A-D / D-A converters, tubes, etc) then make sure you have lots of air between units, too.
Oh, I make my racks out of wood -- so warping is an issue too, not just imprecise rack unit measurements. It probably won't affect you if you're building a small producer's rack. But for a wide or tall (4' +) rack you'll definitely see warping.
Post some photos when you're done!
Cheers,
Johann
EDIT: One thing I forgot. I never use rack rails. So my measurements above are for wooden housings, not traditional racks. I don't like rack rails, I don't like rack rash, and I don't like the added complexity in sorting out ground loops when metal chassis' are all attached via metal racks. I figure eliminating metal racks will eliminate some of the complexity in my not-quite-perfectly-wired studio.
Instead of rack rails, I place 2-4 wooden spacers between each unit in my racks. This ensures breathing room for hot equipment, too.