first to help you out the parts you are refering to are Valve Guides
and valve seats. There is not a bushing in the guide it has a press fit when installed into the head, you heat the head and freeze the guides and install them like so so when the contracts when the head cools down it holds the guides in, same principal with the valve seat. Where the spring sits is called a Spring Pad and is protected by a thin steel shim that proctects the spring from digging into the aluminum.
In my opinion, I tell my customers it doesn't make sense to do things twice, if you head has some miles on them just replace the valve guides while the head is apart because it costs alot more to do it again later. As far as the seats, it is highly unlikly that they will need to be replaced, they do not wear like other parts in the head and unless they are nicked for some reason will just need to be freshened.