Hi Joe,
For manifolds, many different situations, many different respected opinions.
After seeing my warped manifold, thinking about time involved to drive 40 miles one way to get it milled, & 40 miles to pick it up, (total 160 miles fighting traffic), with the rear possibly still sagging, I called a vendor first to insure that the new intake "and" new exhaust were "both" made by the same manufacturer & that milling flat would not be required & I could be assured of a perfect fit.
Installed new precise fitting manifold with:
a. Ford designed new gland rings to prevent future exhaust manifold sagging; &,
b. Copper gaskets.
For me, while still working, I prefer to do it right & do it once because time is money .................. but each to his own with all kinds of different circumstances.