I have alway gone to great lenths to make sure no dirt gets to the compression rings. Any binding that would cause the ring to stick during the first startup would scar the cyclinder wall. Cleances are the largest when everything is cold. The old timer I learned from use to keep the cyclinder head area covered at all time cause he did not want dust in there. You are talking about a carbon and oil sludge bath on top of a piston. To what ends? The piston will just get carbon back on it in the first 30 minutes of running. I really do not see the point unless the car has an excessive buildup which is very uncommon on a fuel injected car that is running right.

Pulling the valve out of the head and cleaning the carbon off the back is a job with minumal risk and may actually help the car.

 

Gorn thank your master for his enlightment. I (grasshopper) has been informed on the basic of carbon pistons

 

You may be picturing a sludge bath, but I wasn't talking one. If someone doesn't know how to do this type of thing meticulously, then I spose it would be better off left alone.