I like to use squaring up rulers (square rulers) the size my block is supposed to be. Then I can center it on the block and trim it down - mainly if the edges are just a bit bigger/uneven as needed. That way, all of my blocks are the same exact size. I found a great place for getting the sizes I needed of squaring up rulers (square rulers) plus other rulers and such as well. I was looking for an odd size of ruler one day and found them. They have great customer service.
Off the Wall Quilt It looks like they go to quilting shows as well.