Which way should the top stitch go in cross stitch?
It does not matter which way. It matters that it is always the same way.
The convention
Most stitchers work the bottom leg from bottom-left to top-right and the top leg from bottom-right to top-left, so the top stitches all lean the same direction. Left-handed stitchers often reverse it. Either is correct as long as the whole piece agrees.
Why it shows
Floss catches the light along its length. A block where half the top legs lean one way and half the other looks blotchy from across a room even though every stitch is in the right square. On a dark ground with pale floss the effect is stronger.
Working in rows
Stitch a row of half crosses left to right, then come back right to left completing them. Every top leg ends up leaning the same way without thinking about it. For single stitches, form the whole X before moving on, and check the direction of the top leg as you do.
