It very much depends on the pattern. At the moment, I am making a project with HSTs that are 12, 8 and 4 inches. I realized very quickly, that I had to square everything up TWICE just to be sure that they all fit together properly.
I always try out a block with scrap fabric and measure to see if it comes out to the dimensions that are specified in the pattern. Saves time all around. But normally, if it is just a simple 4, 9 or 12 patch, I would not bother. I trust my sewing ability. If it is HST or QST then I always cut slightly larger and trim to size.