I usually use them. I usually put the shiny side out only becuase I reason that the head's surface has more cooling than the exhaust manifold.
I'm not an automotive engineer; I only play one on TV.
On FE engines the manifold bolts and manifolds themselves have a much higher chance of cracking because Ford had the genious idea of making the head have very little cooling jacket near the exhaust ports.......usually the super high heat causes hideous corrosion of the upper exhaust manifold bolts and they end up cracking or at least seizing in their holes in the heads.
I didn't realize this was such a controversial topic. My understanding was that you could machine the manifolds to have a nice plane surface and then leave out the gaskets, but short of that you had better have them in there. I've always had them on in 20 years of driving early Mustangs (all small blocks), and haven't had any issues, so I will take Sharkmouth's advice and put them on shiny side out.
On the daughter's we did the following. Surface the manifolds to make sure they are smooth and flat, scraped of the freash paint on the head surface where the manifold would mate, and no gaskets. 5 years 45,000 miles no leaks.
Word on the street is do use gaskets with headers.
If only the windshield and rear window would seal so easy ::