I don't always change brushes after cutting the comm. If you havent abused the brush (overheated, worn out) there is no need. Just keep them clean. The second worst thing you can do to a fresh comm is run break in new brushes on it (the worst is to run them without being broken in). If I need to change brushes, I cut the comm, break in brushes, recut the comm, run.
I replace brushes when they are worn, probably when 1/3 - 1/2 of the new brush length has been worn off, or if they are discolored due to overheating. You can see some rainbow colors when this happens.