Bearing failure is common in nitro engines due to the extreme temps and grime they are exposed to. I'm not referring to total failure, but seal failure.
The worst part is diagnosing the problem. It can first seem like the carb is flaky, causing you to change the tune and losing a good setup.
I would not suggest removing the bearings when doing an initial tear down. Regular maintenece should include cleaning them by removing the seals and using a spray cleaner to clear old lube and dirt. Then use a quality bearing lube and reseal them. Be careful when reseating them into the block that you use equal pressure all the way around the outter race to avoid binding that may damage the bearing.