I find with careful cutting, sewing and pressing along the way rarely do I have to square a block. About the only thing I square is half square triangles. I make them larger and then trim.
I will use starch at times before I cut. To starch it I lightly spritz the material and then roll it up and let the starch absorb into the material. I do something else while the material is mellowing. The material is only slightly damp when I press. I always press my material before I cut it. I use a dry iron as I don't want to add the moisture that steam gives. I press each seam as I do it. This allows me to have very sharp intersections and I think keeps things more true.
When cutting my fabric I try to keep the grain as true as I can. The more off grain you are the more likely the piece will distort. Some shapes other than squares and rectangles are going to have some off grain sides which is one of the main reasons I starch which will stabilize them some.