in my experience, i feel male/female is best.

growing up my parents had a female shepherd and a female carin terrier. these two grew up together and were never apart. they were like bonnie and clyde (truly bonnie and sarah)

one day, however, something just snapped and they went at it. close to being fatal they had to be separated forever. so they were locked to separate perimeters of the house. accidentally, they were let out at the same time and bam. it was like they totally remembered. they just attacked each other. unfortunately the carin terrier didn't survive. lungs were punctured and we had to put her down.

i have a 6 mos old female sheperd myself now and a 10 mos male retriever. they've been together since babies and i haven't had one problem. they are soooo great together!

my mom now has another 4 mos old female shepherd. when putting the 2 female shepherds together, there was slight gression shown. so now we watch them carefully.

i think it mostly depends on the breed of the dog and their background. i've heard of instances where people have 2 males/ 2 females and all is well. i tend to feel safer with the male/female combo however.