IMO if you follow what corporate GM recommends in the service manual then they should still honor your warranty. The dealers are adding unnecessary tasks. If they continue to recommend this, ask them to show you where these requirements are in the manual.

30k is not that much mileage. As long as you are using top tier gasoline with cleaning additives (Chevron, Shell, etc.), you do not need the additional fuel injector cleaning. Checking spark plugs for deposits is a good indicator.

Just my 2 cents.