There are table widely available to figure out the ideal spine for the archer/bow.
I don't quite believe in the "bare shaft shooting" because as M'vy said, there are defects in shafts, vanes, nocks, points and even weight difference would affect the grouping. The best thing I find is to fetch them and shoot. Number your arrows and check which ones are off the group.
I believe spines above or below the ideal wouldn't make much difference, at least for barebows or olympic recurves unless you're shooting in the olympics.
I shoot barebow for Field and compound for target and, as a comparison, I don't spend nearly as much time working on my field arrows then I do for my target arrows (specially the indoors).
There are also little tricks you can apply to your arrow tuning like heavier points with smaller vanes, shorter or longer arrows (always leaving some safety clearance) and so on.