So, with all the hard and fast rules about being sure our quilt pieces are on "straight of grain", if we cut all our fabric with rotary cutters we know for sure that that will not be the case. However, I think that with the quality of fabric now, as long as it is "close" to being on grain it will hold shape pretty well. I think fabric is probably much better woven these days and less likely to distort. At least I hope so, as I have made many quilts using only rotary cutting, and would hate to think that all those little pieces in all those quilts are going to distort and shift as they are used and washed.
This whole topic may be making a mountain out of a molehill!!! If you want to tear, tear. If you want to rotary cut, do that.