Are the squares already cut? Or are you piecing strips, then cutting into squares?
If the fabric has not yet been cut, I would recommend heavily starching the fabric. This prevents stretching and distortion while you piece.
If piecing strips, they are more stable cut lengthwise rather than the most usual crosswise. The lengthwise grain is more stable, so the strips will go together with less distortion. If you can heavily starch before cutting strips, then it won't matter whether the strips are cut crosswise or lengthwise.
If you are piecing already-cut squares, I'm not sure how you could keep track of lengthwise/crosswise grain unless the fabric has a directional print.