I agree with the starch suggestion but you also have to be careful when starching that you don't stretch your fabric. Press rather than iron and don't have it very wet. I would hesitate wetting my finished block with either water or starch to bring it into shape. You might get it to fit in one place and be worse someplace else. In the future do your starching before you cut. This will help stabilize the fabric so it isn't so pliable.
When I am assembling things I look to see what I need to do to make points match, seams match etc. Those will be noticeable. A seam that is not a true 1/4" that happens when you are matching points is not noticeable. Ideally you want things to be square and the same size but if one is a little off then go for what is going to show. If squaring your blocks chops off points then don't square your block and ease it together instead. You want your end result to lay as flat as possible with all the points and seams that are supposed to match matching. Often times that takes some fudging here and there.