first off----there are no real rules---do what works for you! what you are wanting to do is what most (square in a square- square on point) instructions will tell you to do. the only thing you ever really need to be aware of is bias edges will stretch- so when working with them starch helps- pins help- slowing down & taking care helps-
people who work alot with scraps seldom pay any attention to grain lines- same goes for appliquer's- you just need to take care with bias edges- and make it the way you want to-and it works for you.