I do because a long time ago, I made a throw & nothing matched in size.Had to cut the blocks down,add sashing etc.I prob had to cut the sashing down too, as I was pretty new.
There are several reasons for making a sample square.With blocks that all have to be turned/sewn in a certain order,you check each against the sample.Think pinwheels & log cabins.
Checking the block size can save hrs of work.
If the sample is good to go...it's stuck right in front of my machine.If not, it goes into my pile of orphan blocks :-)that will someday be sewn together & given away.
The main reason being sure I want to spend the time/work /fabric making that quilt.When it comes down to the nitty gritty,often times the answer is no.The block speaks to me & that alone has saved me buying a ton of fabric.You would think just seeing a pic of a quilt would work..& sometimes it does, but making a sample block works best for me.