if you're asking whether to cut your strips across the width of the fabric (before slicing them into triangles), or down the length, i don't think there are any rules. you'll end up with fewer scraps, i think, if you cut across.
either way, whether you cut down to the right or up to the left, unless you plan to throw away half the triangles you cut, you're going to use both diagonals. pick the way that's most comfortable and gives you the steadiest hand with the ruler and cutting tool.