Last time I did my valve job, I figured I'd replace the gaskets because they were weeping a little. Naturally with the new gaskets they leaked a lot more.
So, I doubled up the gaskets. If I'd known I was going to double them up I might have tried to reuse the old ones, but gaskets are cheap compared to the pleasure of not having motor oil dripping on my garage floor (from the valve covers, at least).
What changed to cause the new gaskets to leak more? I dunno, I was gentle with the valve covers, perhaps the gaskets weren't seat correctly or maybe they needed some oil to expand..
One day, maybe next valve job, I'll try monte's glass trick and see who's warped.