When and why do you upgrade the valve springs on your heads?

 

Usually when you get cams then you would upgrade depending on the new cam lift specs.

 

And/or for those who are revving way higher than OEM redline. Try revving to 9000 RPM on our V8s (example here, dont take it too seriously). You're probably going to experience some valve float. Stiffer springs may help overcome this but you will be increasing friction between the camshaft and the valve springs.