Usually, it's not actually printed off grain, but has gotten off grain through the processes it goes through from printing to moving onto a bolt.
I will try to get it back on grain through stretching from opposing corners. I detest stripes or patterns that are meant to be straight by are not when they are in a block. Mostly, though, I prefer on grain bc it frays less, so less of a mess throughout the making of the blocks and top.
Stores do not often cut on grain; the method that most quilters use - lining up a selvage and shifting until the fold hangs straight - doesn't get the grain straight. In the old days, when a snip was made, then the fabric torn, it was back on grain. Now, people cringe when anyone says they do this and talk about loss of fabric due to the torn edge. Well, if I have to trim an inch or two off of one edge to get it straight, I'm losing more than I do from that torn edge.