Many blocks cannot be squared up at the end, you will lose your points, seams won't match up to your other blocks, etc.
Squaring up a nine patch for example, makes the outer blocks smaller than the inner.
Squaring up blocks with flying geese in them will usually make you lose your points.
Squaring up a drunkards path block can make it difficult to line up the circles when you go to sew them together.
You need to be careful too when squaring up your blocks, taking care to remove extra fabric evenly off of all sides. Just removing it from two, may leave you with a lop sided looking block. The same with taking more off of two and less off of the other...
Take a good look at the block while it is under the ruler. Look at what the final appearance will be after the cuts. You may want to resew a few of the seams instead :wink:
But, there are some blocks/patterns that are specifically made to be squared up as you go. :D
It is usually better to try and be accurate with each component of the block, fixing them as you go. It will make putting them all together easier too :D:D:D