The savings in labor is due to the fact that you do not have to be up with cattle during the night as well as you don't have to bring ever calf to the barn during a storm.  Obviously there is a difference in climate for producers, further north winter may last longer.  Generaly by May cold weather and snow storm are over here, with the exception of a few years ago.  A good grazing management sceme will enable a person to be able to graze through most of the winter feeding only through the bitter cold and snow cover that can not be grazed through.  The other consideration would be the number of cows a person is taking care of.  I would be realy comfortable calving a large number of cows myself from April on, you can see the light at the end of the tunnel.  I have also calved 200 head of cows from Feb to May and thought that the light was never going to come.  I calved that set of cows alone, I am glad I did not have a family because while I would have been in the same house I would have never seen them.  I was up all night and had to feed and pair out and tag all day long.  Even though we only got a few calves each day, if I missed one they froze to death.  Regardless a set of 2yr old heifers require more care than a cow for obvious reasons, no matter when they calve.