You want headers with AT LEAST 3/8" flanges. Keeps them from warping. Another tip is to NOT USE any type of soft gasket. They will warp!!! On the head you have to use some type of metal gasket that doesn't crush. If the gasket crushes, it will warp the flange. Most cases I use either MLS stainless steel, if available, or a thin film of high temp red RTV. This only works with 3/8" or thicker flanges tho. Anything thinner is a POS header.
Some header manufacturers weld a bead and then machine it flat to get a "seal surface" for the gasket. That just prolongs the inevitable. It will warp from the continuous heating and cooling cycle. If the flange is thick enough, it won't warp. Look at some of the 1/2" cast iron stockers, they came from the factory without gaskets..........