When the valve diameter is big, and has a long stem. Like the B1 M/C stuff its a almost a must.
But for most engines turning below 8K with a moderate valve size. You wasting money IMO

[color:"red"]65 Hemi Belvedere coming soon [/color]
[color:"#00FF00"]557" Indy engine 1.07 60ft 144mph in the 8th 2100 lbs package [/color]