The little details add up across the span of a whole quilt. Check the unit size and the finished block size. It's easy to correct and re-do if needed.
I like to sew 4 finished blocks together and square those as needed to make the quilt. I seldom sew rows of blocks together in long strips. Really important blocks are stay-stitched and then pressed to keep the shape. Stay-stitch around the quilt and all the borders before sewing the borders on. Press again after stay-stitching.
Can you tell that I'm a fan of stay-stitching? Check the edge of a strip of fabric without it. Stay-stitch and press the edges, See how they are flat and smooth?
I haven't entered any shows so I don't know show standards but here are a few tips for the final square-up after quilting.
Check the border area to make sure it is still the correct size. Dampen with a light spray and block to the size needed. I use two large rulers plus the 24x6 rulers to check corners and edges first. There are metal 48 inch rulers for about $8 at the hardware store. These make great helpers too.
I like to cut down the quilt about an inch larger than needed first. Then go back for the final check and cut. It's easier to manage without the excess batting and fabrics. Good luck with your quilt and the show.