That's a good question, I suppose I could have made that more clear. So basically, just because I have listed a part in this guide, does NOT mean that you have to replace it.
This might not be the best practice, but I have listed every part for all of the main components that are detached at any point during maintenance.
I have also listed parts in which the torque can be verified without too much trouble and any part that has a limited lifespan that's associated with one or more of the main maintenance components.
I feel this was a good decision because anything that is removed from the vehicle for maintenance has the chance of being damaged or getting lost.

As far as gaskets go, rubber gaskets, like O-rings can be reused one time as long as they're properly lubricated and changed at the recommended intervals, but the metal, crush washers should really be replaced any time that bolt and washer are removed.