Welcome from Missouri!
Lots of good advice here - obviously there are a lot of ways to get your consistent seam allowance. Try several until you find the one you are most comfortable with. :-)
The scant 1/4" allows for the size you lose when you turn and press your seam. Doesn't have major impact on larger pieces/fewer blocks - but....on smaller pieces/more blocks the difference is compounded.
Now for the cutting - when I was experienced enough to be really frustrated by my blocks not being right, I realized it was my cutting. My ruler slipped or I didn't measure right, or something.
Here's what worked for me: cutting at the right height (cutting table/kitchen counter) - much less strain and more accurate cuts; good lighting! even in daylight I use a lamp over my cutting table; after I have cut, I turn the strip around and see if I need to sliver cut. Those slivers really add up.
Have fun!