I understand where you coming from and the original thread has to do with valve cover screws, but the *general* statement I made about aluminum threads possibly galling when the material is cold holds true whether it be on a car, an industrial piece of machinery or something around the house (that would experience cycles, weather, etc.)...Sure it may be a less common occurence with a valve cover screw vs. something that comes in contact with coolant, let's say, but doesn't mean it's not a possibility.
I was not suggesting you *have to* have a warmed up engine to remove the bolts, but your chances of the threads getting messed up will diminish a great deal if you do. I've removed them and installed them numerous times when it was cold and haven't run into the issue, although I generally always use anti-sieze even on my plugs, so I wouldn't expect any problems. Working on something that is old and has been thru numerous cycles just opens up more possibilities of things going wrong and I'm not one to risk turning a simple projetc into a headache. Generally speaking, you can feel how it's coming out (or maybe even hear), so at that point you can make the determination...Even on my nissan truck (only had 18k miles on) when I pulled the oem studs out of the intake manifold (bottom is alum.), I ran into some issues, where as when I warmed it up (on dyno day), it went much better. Finally, I like to use stainless hardware quite a lot, so this holds more true (possible galling; using anti-sieze) vs. the oem steel, but even so.