One is to check in the code as soon as the change is complete, so that your co-workers can see the change and to minimize any merging that has to be done when putting a build together.
The other is to not check in any code until you are certain that you are ready for a build, so that if a problem is found, you can address the issue as quickly as possible.
These are really both correct. You should check-in as soon as you have code done. Your check-in should always build.
That is the entire point of Continuous Integration and stuff like gated check-ins. You have a build step to make sure that you didn't break the build. You have a suite of unit tests to make sure the code didn't break stuff. You might have a set of smoke tests to make sure the code works on a real environment. All of that provides abundant confidence in #2, so you can abide by #1.
