See - I think that is an excellent reason to rip across the grain. I do not want to put warped fabric into my quilt! Fabrics may look okay on the bolt and then turn out to be "warped". Ripping it will reveal any problems. Prewashing will help to straighten things, and you can even try stretching and blocking if the fabric is worth the effort, but it's good to know if you have bad fabric.
It's not just Walmart. I purchased a couple yards of Red Rooster fabric from the quilt shop recently - a large scale plaid - and it was printed TERRIBLY off grain. It looked fine on the bolt. I ended up cutting it along the print (off grain) and using it to line a crafty project, but I am glad I ripped it and didn't end up using it in a quilt.