Slag / Tom,
All courses should have ratings for the front 9 and back 9 as well as for the full 18 holes.
For example, see
Pebble Beach RatingAdditionally, ratings can easily be computed for any specific set of 9 holes on the golf course.
At Hannastown (see My Home Course), when the new 9 was built, the original 9 was used for holes 1,2,9-12 and 16-18. Since these were the 9 holes that were next to the clubhouse, many people played them in the evenings. I had the WPGA calculate a rating for those 9 holes so that scores could be posted.
The following assumes you are using a computer in an association that uses GHIN. I assume other handicap providers in the US have the same capability.
When you go to the computer on your home course, you will be asked if you played 9 or 18 holes. If you pick 9, you will be presented with the 9 hole combinations that have been rated for your course. You select the one you played and put in your score. When you have two 9 hole scores saved, they are combined into an 18 hole score. If you look the printout of your scores, these rounds will be marked with a "C".