The flanges on the headers should be made to match the gaskets, which match the heads.
IF you have selected a camshaft that maximizes the flow characteristics of the ports, then match the gaskets on the exhausts.
IF instead you have selected a "sane" street cam, then don't bother.
It's far more important on the exhaust to match the size of the tube to the flow of the exhaust port.
If the flow numbers on the heads show say 330 at .630, 220 on the exhaust and you put in a hydraulic lifter cam that is .580 lift, again, don't bother. No matter what you do, the heads aren't working anyway.
It's pretty difficult to maximize the flow of the headers at the flanges. All sorts of compromises had to be made just to attach the tubes. Many turn the wrong way or the radius is too tight, etc.
Even if the builder got some of them right, usually half of them are wrong.
Matching the gaskets? "Much ado about nothing". Save the money.