I don't use the gaskets made for headers.

Like the previous poster, I make sure that the headers don't have a crappy bead of weld that's not flat.

Then I put on standard exhaust manifold gaskets, with a bit of anti-sieze.

I've even re-used 'em occasionally.

Seems to work for me; like I said above, it's probably just that I've never been told why it won't work, so it does.

BTW, I've had problems with header gaskets blowing out, even with filing the flanges.

-Bill