Even if your tension is uniform, when you crochet you will use slightly more one one square, and slightly less on another. That is the very nature of "tying the knots" used in crochet. As long as you have the same number of stitches in each motif, you will have no trouble with assembly, and the first time you wash it in cold water, the threads will relax, expand, draw up, what ever, and everything will be even looking. I crocheted for years until carpal tunnel tried to get me. I quit crochetting and 17 years later, I still have complete use of my hands. I do miss it.