I iron each seam in my block as I am sewing it and then square the block up after finishing it. I do this on the first and every block. This lets me know very quick if my seams need to be a little bit bigger or a bit more on the scant side. Once you get the first block right, it should be much smoother sewing and squaring for all the rest of the blocks. I don't use any starch, but neither do I wash my fabrics first. If I did use fabric that was washed and soft, I would use some type of spray starch or Best Press.
I have tried squaring up after I finish all the blocks but found it very confusing and if you have anything with points, you end up losing some of them. For me, squaring up from the beginning is the only way to go.
I have a square up ruler that is 16 1/2". If the block is that size or smaller I can use it or one of the multiple other square rulers I have. If it is bigger, then I use my cutting board lines. Small amounts of difference can always be adjusted in....small being 1/4" to 3/8" on a 12 inch block. Any thing bigger tends to try to get tucks. If you have one block slightly bigger than the other, always put it on the bottom when you are sewing. The feed dogs are what pulls the fabric through, and they will help when you need to ease fabric.