The thing to understand with bias is that it gives you a great deal of somewhat temporary stretch. After cloth has been washed and dried (especially in a hot dryer!), it will usually want to return to its original shape. So if you take, say, a half square triangle and iron it so it is in more of a diamond shape than its original square shape, then you trim it to be a neat square, and then you sew piece it with other quilt pieces, at some point down down the line (after laundering), the cloth in that half square triangle can return its original orientation and you have a shape that is a bit diamond shaped sewn into a spot that is intended for a square shape, so the fabric may not lie quite flat. Now, if you make quilts that end up with the crinkled look (due to how the batting behaves relative to the quilt top after the first washing), it may be practically impossible to see that my example is not lying flat.