The recent trend is to tune to the nth degree, and in truth, most folks don't have form good enough to do it. I'd vote for form being the most important factor. Have been shooting sticks for more than 50 years. One of the things I've discovered is that most bows will shoot a fairly wide range of spines well, on the overspining side. They don't handle underspines as well.
One of the things Howard Hill did in his shooting demonstrations was collect an arrow from each of the shooters and proceed to shoot those arrows, which were obviously not matched to his bow, into the bulls eye one after the other.
Now cameras didn't usually focus on arrow flight back then, but I suspect it was pretty good as well. Sure, if you strip an arrow down and put a big, bulky head on it, it has to be matched perfectly to the bow to shoot straight. But short of that extreme, most arrows that are spined reasonably close to the draw weight at the shooters draw length will shoot quite well, better than most are capable of shooting.