Just went through this myself, except for new heads. I'll start off by saying that if you are modifying the valve lift or are rebuilding a set of heads, have the studs pulled and screw ins installed instead.
I was told that as long as I keep the lift on my motor under .450, there is no reason to pull the factory studs. Again, these are brand new GM heads, and I am going to be running a roller cam with .429/.452 lift. Just a FYI, since I didn't want to spend themoney to have it done if I didn't have too.
Dorian
My K5 and Chev/Olds tech/links page: <A target="_blank" HREF=http://yeagerd.home.mindspring.com/index2.html>http://yeagerd.home.mindspring.com/index2.html</A>