I mostly do cross-grain except for stripes or curved edging. However - one of the reasons for using biased binding is that it will last longer. When cut on/cross grain, the fold as well as the edge of the binding hinges on one or two threads. If one of those threads wears and breaks it can lead to a split in the fabric. If cut on the bias, the folds of the binding cut across the threads, don't align with them, and any break of a single thread is just a small hole and less likely to "rip". That being said, and apart from decorative stripes in binding, using a biased binding for children's quilts might be a good choice because of the wear they are likely to get.