I suspect that about the only way one might burn a valve with open headers is if the engine runs extremely lean with the open exhaust...and, that it would likely affect a more highly modified engine (more sensitive to af changes) if at all. There is virtually no way for cold air the "rush" into the headers especially considering the amount of heat headers retain and the fairly small pipe diameter. If you want to do it, keep an eye on your plugs for any telltales.
Btw, heard the tales of burning valves, but have never seen it, even in my days of auto parts and mechanic work. I've run open headers and if you've gone to the trouble and expense to install them you may as well give it a try...they're too loud for me!
Underestimate me, that will be fun.
GREEN WORM RACING--since 1959
These are my opinions...YOU are responsible for YOUR decisions and YOUR outcomes...AND, my not arguing with you does not indicate agreement with your opinion.