These are all good suggestions and I have one more. If a particular seam really just won't match up, take it apart and fix it by either making it deeper or less deep depending on what you need. You can also deepen or let out the seam allowance on just one of the blocks instead of both if that is what you need.

I agree with the others' comments about taking time to pin accurately. The first quilt I made though, some of the seams didn't line up AT ALL, despite squaring and trying to pin accurately. Baffled both me and quilt group tutor until I realised what I thought was a 1/4" foot wasn't QUITE 1/4" (cheap machine that a neighbour was getting rid of). The pattern measurements had obviously been calculated to account for seam allowance, so that the pieced blocks would end up the same size as the single fabric blocks, but because of the inaccurate foot, they didn't, so the rows didn't match properly.

Nest your seams and put a pin in on either side to hold it in place.

That is a very good video, thanks PaperPrincess.

I have always been a "pinner". But the main thing I have found is to press everything. Make a little cheat sheet of your block with little arrows telling you which way the seams should be pressed. Then when you have to put the rows together all the pressing is the same for each block to nest together. Then all the rows will have their own pressing order so their seams will nest together. Hope this helps a bit. Just keep at it, you'll get there!!

Love squint would that be the same as squish (crooked).

I would recommend to beginner quilters to watch Eleanor Burns and/or buy her books. She is a wonderful teacher and has a really good method of quilting that makes things go faster and easier.
Heres a tip for sewing two patches together that I learned from Liz Porter when she made an appearance on the F&P show recently: Align your patches and make sure that the seam allowance on the back of the patch is going up toward the needle instead of down toward you. The feed dogs will push the seams together and your seams will match every time. Just make sure you have them together before sewing and let the machine do the rest. Works every time for me. I hope that makes sense. You might just have to make a sample 4-patch to see what I mean. Gina

the problem is probably already in your own words "I square up my blocks"
If they need squaring up there are several solutions.

The fabric needs to be pressed flat before cutting (I also starch as I fall in the prewashers camp)

Cutting has to be consistently correct. An inch is an inch is an inch, BUT different brand rulers have different sized lines on them. Do you consistently cut on the line, short of the line, over the line? Even a 1/16 off will add up when you put 30 or 40 or 90 blocks together.

And piecing...You have to know where the scant quarter falls using the foot you use.

the easiest way to do this is making sample 9 patches using 1.5" strips plus the steps above.
Often the first three strips sewn together measure correctly across when pressed...
but once cut and reassembled in the 9 patch the block will suddenly comes up short of true. Because those crossing seams eat up a bit of fabric behind your back :) Keep making them until it all falls right.
I have a quart baggie full of these 9 patches, they remind me no one knows everything...

I promise you, if you revisit your basics (including the squint advice above) you'll never have to waste time squaring up blocks again. And I never pin until I have the long assembly seams to do.
Best wishes and happy quilting!

when squaring blocks up you must trim them all exactly the same way, otherwise any pieced seams could be off. always use your 45 degree line on your ruler and place it the same on each block as you trim.

IMHO ~ cutting, accurate 1/4" seams, pressing, check piecing as you go and pinning ~ when final block is competed, one should not (hopefully) not have to square up the block. One or ALL could mess up a block.