Yea somewhere on the internet but don't remember where. It's hard to find any literature to study this. I'm afraid if did more than a few degrees you would run into bolt hole issues (which can be fixed) and geometry issues. I run a sport mod class so gotta stay within rules. Pushrods have to be stock lengthYou never know .
I'm curious where you heard it ?
on the inter web , a bench racing sesion or a engine bulder ?
I personally don't see angle milling .020 or .030 off actually helping .
It would give a slight angle too the valve , Likely less then one degree .
4* or more would surelyhave an effect .
Only allowed a cam .310 lift or less no bigger valves or porting. Just looking for a edge within the rules.Myself i wouldn't worry about that .
Bigger valves , port job or camshaft if they are within the rules .