I do a light starch before I cut the fabric when I am doing the first ironing, just to help get wrinkles out. I will spray a light mist of starch on the fabric, let it stand for about 15 seconds, then iron. Sometimes my iron will stick just a bit, but I just move it around and it is fine. You have to keep the iron on the fabric until the starch dries completely, that is the only way it will stiffen. If you have a particularly wet spot, just move the iron around over it a lot so you don't burn your fabric by keeping the iron in one spot too long.
Then I will cut all my pieces and start sewing them. I will sometimes do a light spray on the seams before I iron them open. Sometimes I don't because the starch from the pre-cutting is sometimes enough. Once the block is all put together I give it a final light mist of starch, iron it flat and then square it up.
If you are working on a project now, you can use starch at any stage, but at the very least you can starch your final blocks to get them nice and flat and be able to square them up nice.
Cheers!
Rachel