I learned my lesson with field curvature when I shot group of managers with Canon EF 24-70 at the wide end, focusing on the center. At f/16, I thought that I am safe from FC, but few pals who were forced to be on the frame edges, weren’t happy with their blurred and distorted versions… and thus I lost great client. (It was beer brewery, whic translates in many events with free beers, nice chicks, etc :-()
In some critical situations (it usually works for landscape), you can try focus stack (5 shots should be enough) for outstanding sharpness across the frame. In other situations, you might find focusing on the edges at f/8 and smaller sufficient for good results.
I don’t have Loxia any more, but I quite liked it when I has.