The folding method of the quilt block is used when it measures equal on all sides but still a little larger than needed. If it does not measure equal on all sides, I use the ruler only rather than folding the block. Also, the block should already be pressed before beginning the squaring up. If you do not feel comfortable using the folding method, then by all means use the measurements on the ruler instead. I just prefer this method after years of squaring up blocks. There are different ways to do anything. Find what works best for you and go with it!
Rulers used can be found below:
9.5 inch ruler
https://www.sewcharmingquiltshop.com/shop/c/p/Square-Up-Ruler-95x95-2012-Quilt-In-A-Day1-x69929815.htm
12.5 inch ruler
https://www.sewcharmingquiltshop.com/shop/c/p/OmniCraft-Square-Up-Ruler--Limited-Edition-x69929826.htm
www.sewcharmingquiltshop.com