Having done this myself, I would never recommend replacing this system piece by piece. Perhaps if I crushed something accidentally and knew for sure the age of the components was acceptable I'd leave it and only replace the ruined part maybe....

The work to get in and replace things is just SO crazy, it's NOT worth /NOT/ replacing everything.

I did it myself (in the cold no less yikes!) and I replaced every vacuum hose I came across, and cleaned the throttle body (+ new gasket) and IAC. It was a long day (~14 hours in total including the cleanup - I was in someone else's garage... boo!) but I was moving slowly that day and it was worth every penny.

As I said, having done it myself, I can say I now understand how much oil/dirt/heat these hoses are subjected to, and combined with age, this is why they fail. If you're in there already and you have the intake off and the battery and breather box out to do the PCV, you may as well take the time and clean everything up and replace everything you get your hands on (the disposable parts anyway).

The part you're really concerned about is the oil separator, which is bolted to the block below the intake, and is the hard part to get to. If you're worried about the PCV, you need to replace that, and once you're in there for that you may as well do the rest of it (minimal) - the rest is cheap insurance.