I try to follow the grain as much as I can when cutting, this is acceptable if the design does not have lines or stripes. If the fabric is printed right and the lines follow the selvedge, you won't have a problem with cuts going askew. When you sew seams the fabric is held captive and I haven't found it to pucker or get frayed if cut a bit off-grain.
If you wash your fabric first and get rid of the starch or whatever they use to stiffen it so it looks firm when buy it, you will be able to tell if it is printed according to the grain.
I have found some fabric is pulled unevenly onto the cardboards and there is nothing you can do to straighten it and I only hope there isn't a definite pattern to the design.
We used to pull the fabric diagonally to make it straighter, that helps if you find once you cut it off a bolt and take it home, the selvedges don't meet. Has anyone else found that to be true? The clerks don't cut on the grain or even try so it is best to get a little bit more than you need.
Carol J.