My sanding method is to mark the frets with a texta, and then sand side to side using a spirit level with 360 grit double-sided-sticky-taped to it.
By "side to side" do you mean across the width of the fretboard?

If I'm interpreting that correctly, sanding lengthwise down the neck (or up) perpendicular to the frets, is the generally accepted practise. I'm not saying that can't work, but I've not known of anyone doing it that way and can see how it could lead to issues.

If I've interpreted that incorrectly... ignore this post!