I became aware of this issue a while ago, when it generated a good discussion on a prior thread. I understand what you are doing when you refer to diagonal stropping. With an X stroke, when you reach the top of the strop, you move the razor slightly across the strop before you flip it and bring it back down. It's this slight diagonal movement that let's you make the other side of the X on the way down.

My sense is that in terms of the edge, it doesn't matter whether you use the diagonal or X stroke. Since the last thread on this, I've been alternating the diagonal and X stroke on consecutive shaves and have not noticed any difference. Use whichever stroke you find more comfortable. For newer users, eliminating the lateral movement at the top may reduce the likelihood of nicking the strop.