It is almost impossible to replace the cam bearings while the engine is in the car, and get them in the correct position. Cam bearings generally do not wear much in a properly maintainted engine. If the cam bearings are worn, then the whole engine probably needs to be rebuilt. During a rebuild it is imperative to replace the cam bearings, as most engines have a groove in the case around the perimeter of the bearing shell. This groove can only get cleaned out if the bearings are removed.
In summary, if you are just replacing the cam do not attempt to replace the bearings. But do look at the #1 position bearing, the only one you can see unless the engine is disassembled. If this bearing is in bad condition, like scratched and gouged, then all of the cam bearings should be replaced.