I have a vintage quilt that used sugar sacks and then appliqued butterflies on the blocks. Some of them have printing still slightly visible. One thing that the level of coarseness depended on was the original use of the sack. It makes sense that sugar and flour sacks would be better if they were tightly woven. Animal feed sacks could get by with a coarser weave.
At our quilt documentation day at the museum a few weeks ago, a family brought in a double wedding ring quilt where the backing was made out of flour sacks. They were not printed with flowers or such, but had the original words and trademark picture completely clear. With a little research a person could maybe get an idea of the date of the sacks, because the brand name of the flour and the mill would be unique. Even the font of the brand name could be a clue if you could find some of the mill's history.
This is what makes quilt research so interesting!!