[On a steel string guitar] If the neck is to be set at an angle to the body, and the body is arched to match that angle so the fretboard can be glued to the top, then why does everyone draw a headblock with a 90 degree angle between top and side? It seems to me if you do that, when you glue the fretboard down, you’ll be mashing it and the soundboard against a block of mahogany that is on a different plane. The obvious solution to me is to angle the top of the headblock. But, when you discuss fitting the neck to the body, you stress the importance of adjusting this angle at that time, so it seems the obvious solution to the above problem isn’t quite possible. How do you resolve this?
Your perception is correct, as far as it goes.
If you draft everything out, side view, on paper in full size, you'll discover that for the action to be optimum and the saddle height to be optimum, the neck needs to be tipped back by a small amount, relative to the plane determined by the perimeter of the soundboard. Thus the end of the fingerboard must rise as it pivots upward, beginning from the point where it meets the front edge of the soundboard. The nut end of the fingerboard rotates downward, while the saddle end of the fingerboard rotates upward, pivoting right where the neck meets the body.
However that rise, measured over the width of the headblock, is exceedingly tiny--barely a 1/64-inch. Indeed the fingerboard-end keeps rising to its terminus and the soundboard must also rise to meet it--just above the soundhole. Thus the upper transversal face brace must be arched about 1/16-inch--generally enough to raise the soundboard under the fingerboard end to just reach it--on a 14-fret guitar--or around 3/32-inch on a 12-fret guitar.
What you propose, angling the top of the headblock to account for the rising fingerboard end is technically correct--assuming that you've kept the rest of the headblock strictly perpendicular to the workboard. But that's not the only way to go about it. If you leave the headblock square where it meets the soundboard, it will drive the front of the soundbox to a small tip-back relative to the workboard--which should match fairly closely the amount the heel would otherwise have to be trimmed later to achieve the required tip-back of the neck.
So take your pick--slant the top of the headblock by 1/64-inch and expect to trim the heel later to set the neck angle; or keep the top of the headblock perpendicular and let the headblock naturally tip back by 1/32 from perpendicular (to the workboard) and dispense with the heel-trimming neck-set procedure later on.