Been a while since I've set up a hewing yard, but my trestles always have heads hewn flat, no notches. They typically have inclined planes on the pile side to roll the log up to the desired height, (for me waist high) then a flat several feet wide, both so you can occasionally double dog and so you can clip off the chewed up end, as preferably the face being hewn is just hanging over the ends, this to minimize the chewing and max head life, and avoid the potential that the head will deflect the ax in some unanticipated direction.
That first face does require you dog off well. (I usually double dog in both directions) I have a nice set of “Danger dogs” the chisel point being at right angles to the spike point, they are desirable because they don't chew up the heads on a long term set-up, but hold a little danger in that there is no way to put them down without a point sticking up, but this is almost a non-issue because they stay pinned to the head unless the weather turns foul. When I need more hold some 2X2's and a few 20d's work fine. (some say better) I trig off that first round side with a few juggles driven under like chocks, and just check the plumb & horizon lines with a whiskey stick with some regularity or if I think I see movement, and just tweak it back with a peavy if need be. But in my experience it's not all that much of an issue.
And once you roll the first side hewn down it is pretty much dealt with.
However, I don't tend to hew sides opposite unless I'm working a really long and elastic stick and I see snapping two parallel lines at once as advantageous because some effort has been spent to condition a stem to strait. The reason for this is twofold, I see greater advantage in rolling that first flat side down to the heads and ending the dance, and I like to keep the juggles and the chips on the one side where I can easily scoop them away with the bobcat bucket. They tend to build up quick, and I find it both healthy (helping to avoid both accidents and backaches) and efficient to maintain that optimal waist high altitude.
