Like IrishRose I don't worry too much about straight of grain except on backing. Since your back is the "foundation" of the quilt, a wonk back will mean a wonky top. But if you are tearing to get straight of grain with wideback, you need to
start at a fold and go SLOWLY in both directions (but only one layer at a time). You will be amazed at how off grain the wideback can be at times. I have been in stores that will tear your wideback so that you get your full amount and don't ahve off grain waste. There is also a big fabric store in Lawrence, KS that pulls a thread and then cut along that marker of straight of grain.