I don't think it's possible to get a "perfect" block "every" time.

That is what the square up rulers are for. They keep the diagonal straight for trimming. You want the diagonals to go right through the center square diagonally, point to point. Most square OmniGrid rulers have this feature as does those That Patchwork Place square up rulers. Try it on paper or scraps first if you aren't sure and want to practice.

It doesn't seem to matter how accurate I think I am. If I'm ironing the seam allownance to the dark side, it throws the size off. If the fabrics aren't the same weight it's a problem. Sometimes the pattern isn't on the straight of grain so the patch has some bias to it when I fussy cut it and that also throws the size off. Starching doesn't seem to help either. I just have a block that is stiff and the wrong size as well.

I've finally have made peace with myself and acknowledge that I am not a perfect quilter and that's ok. If you are planning to compete with your quilt...that's a different story. You might see if you can get an experienced quilter to watch what you are doing and perhaps spot the problem for you.

This will probably win the Dumb Comment of the Day award but, why be square? If you've measured, cut and sewn correctly and carefully, isn't the slight variation a characteristic of something made by a HUMAN? A little irregularity is a nice thing, and in a large quilt with many pieces, tends to balance itself out, doesn't it? Note:

James' Law: A binding will go around a quilt of any shape.

Be careful when pressing your seams of the sewn block....you may be stretching your fabric into a larger size...thus your problem solved in the future

Draw your block out on graft paper with the 2" finished block in the center and the 2-1/4" blocks on the outside and see where you are going wrong. Then make a plastic template that you can put on top of your blocks to square. At this point my husband makes me an acrylic template with those line marks and I then use that to square up the blocks. Especially necessary when you are making small blocks.

I agree 100%. If I'm off a bit on a 9 patch (and I've made a fair few), I just keep going. I don't trim down a 9 patch that is a little off--I just center it on the strip/piece I'm working with. The beauty of cotton is that things can be pulled or shoved as needed to fit. :) If poly is part of the equation, things are a lot tougher.

The reasoning behind squaring up is that it makes it so much easier to match seam lines if everything is square and the same size. If you've got a bunch of wonky blocks they don't go together very well and it just leads to frustration, at least for me. No one us perfect, but it is possible to see a nearly perfect block, or so I've been told, lol.

Sometime I do feel there is too much emphasis on perfection rather than the finished quilt. If each thread is perfected to death, do you ever see the beauty of the whole quilt?

For just the reason you describe, I learned assembly line quilting from Eleanor Burns. Her book, "Irish Chain Quilts" has the method broken down into individual steps. I still use than method because it is just way more accurate for me. Measure at each step, and you are home free. Once in a while I might have to sliver trim, or resew to get the quarter inch, but that is not her fault....lol!