I did like this tidbit about the accents from their podacst (siap)
Speaking on the show's official podcast, Craig Mazin said: "The decision not to use Russian accents was a big one that we made early on.
"We had an initial thought that we didn't want to do the 'Boris and Natasha' cliched accent because the Russian accent can turn comic very easily. At first, we thought that maybe we would have people do these sort of vaguely Eastern European accents - not really strong but noticeable.
"What we found very quickly is that actors will act accents. They will not act, they will act accents and we were losing everything about these people that we loved.
"Honestly, I think after maybe one or two auditions we said 'Ok, new rule. We're not doing that anymore."
That's my take on it as well. In reality, its not like they'd be speaking english anyway, so why have them speaking english with Russian accents? It ends up flowing better with the actors natural accents.