That's a myth. I've run racing engines with open headers for hundreds of laps with no valve damage whatsoever. Maybe if there was no exhaust manifold at all, there might be a little bit of exhaust valve damage, but even then, the engine would have to be run at wide open throttle uphill for miles.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!