swedis,
If you are using wood for your shafts the growth rings (sometimes erroneously referred to as grain) should ideally be parallel for the length of the shaft, the nock, if a self nock, should be cut perpendicular to the growth rings and if an applied nock should be applied so the nock is perpendicular to the growth rings.
If you are using bamboo or similar, then you need to find the "stronger" side, one side is more resistant to bending than the other, in some bamboos, this difference is negligible, the stronger side goes against the bow so the nock is cut or applied perpendicular to this.
Craig
