Stress concentration is always more when you use anything which having corners , e.g.as circular column is more efficient than rectangular column but due to form work costing its not in the regular practice. In circular shape the stress distribution is uniform.
One major reason is that the starting and propagation of crackings is favoured by corners instead of curves (for the same reason circular openings on walls are better than rectangular ones, nevertheless they are more difficult to construct and less practical)