The only time that it maight really matter is if sewing piece a to b was critical because piece c was sewn to both of them as in 2 quater square triangles being sewn to a hlf square triangle in a design, or in applique, sewing the stem first so that the raw edges were concealed by a flower. Other than things like that, if its pleasing to you, go for it. Some things go in a certain order and thats why there are "rules" in order for your block to come out just so. Other times it won't really matter.
Hope this is clearer than mud!