This is an after-the-fact hint, but... taking into consideration that the width-wise grain of the fabric has a TINY bit of stretch in a 1.5 x 1.5 inch square of fabric, and the lengthwise-grain had pretty much NO amount of stretch... if you sewed the *strips* sewing the sides with NO stretch together ....(and of course they really do need to be accurate seams too)... then when sewing the long strips of little squares... you are working with a TINY amount of stretch or give to help match seams........
Yes, when picking up a little square, you can just barely tell which edge is lengthwise or crosswise grain, its a tiny but different response to a very slight pull....
And, because this is after the fact LOL... I have heard of people putting a perhaps 6 stitches per inch basting line inside the seam allowance, and pulling up a stitch when attempting to ease the fabric inbetween two small seams.... .I dont know if this is helpful in an area 1 inch in measurement.. but it might help..... LOL