For me i think. Not until your pc parts are that old and cant handle the modern applications, games, rendering or whatever you do with you unit. Its just a waste of money if you are on the verge of upgrading on the last generation and the new one.
Because just a little boost and another socket for motherboard. and alot worse you build again a new system. Because of the upgrade sickness you have.
I think you should think twice before you upgrade your units. Think of it if it can still handle the tasks that you give it. The games that you play, and the activities that you are doing.
Just remember those details if you want to upgrade and youll be thankful.
Best regards.