As a structural engineer I agree with the grain flow being very important.
If you don't have a hooked barrel assembly and want to remove the barrel for cleaning etc., the through bolt will be easier on the wood in the tang area than a wood screw and will not wear out the wood.
It really depends on what kind of force the tang bolt will encounter. Let's say the tang is fitted perfectly to the square end of the barrel channel and then screwed down. If the wood dries and shrinks because of different moisture contents at the end of the barrel channel and the gun will be fired, the wood screw which is now also slightly looser will have to sustain the shear and will scram slightly in the wood and induce a more superficial splitting force. A screw also has less rounded contact area with the wood and thus will induce different amounts of stress (top of the threads beeing most critical).
A tang bolt is bolted through the trigger plate and will also induce some force to the trigger plate and apply its friction and fasteners. The through bolt is usually longer (about double as long as a screw, has a larger diameter and a smooth surface in a smooth drilled hole. The overall force will thus be spread out on a much larger area and is inmh less likely to induce critical stress on the surface of the wood.
