The traditional seam for quilting is 1/4 inch. I suggest you steam "press" all of your blocks very carefully and handle them with a very gentle touch. Fabric stretchs at the blink of an eye. It usually helps if you pin the blocks carefully at each intersection and in the middle. If one block seems bigger than the next put it on the bottom when you sew two together. Let the feed dogs on your machine ease it to fit the block on top. I have been quilting for more than 40 years and I still have trouble getting the seams to line up perfectly. Fabric is almost like a living thing. Be sure you "press" which is an up and down motion with the iron not a slide which can stretch your fabric.
Good luck.