From the DVD commentary, episode 1:
Douglas Mackinnon: One of the things I wanted to do visually was attempt through the series to always have Aziraphale on the left and Crowley on the right, and we very pretty much did achieve that.
Neil Gaiman: We fail when they're in the Bentley.
Douglas Mackinnon: Because the steering wheel is on the wrong side.
Douglas Mackinnon: If we'd been making this in the America it would've been a lot easier.
Neil Gaiman: It would have been fun.
Douglas Mackinnon: Yeah. But, generally speaking, I wanted to do the old Ant and Dec thing. Which, for international audiences, And and Dec are the UK's most famous television presenters and they always stand together with Ant on the left and Dec on the right. But I that in this case it would just help the audience through visually, just by always feeling they knew where they were.
Neil Gaiman: Yes, and you always know something odd is going on when they're moving around.
Douglas Mackinnon: It's one of my great pleasures in life that David and Michael now, when they do interviews, and they're not in this order, they swap.
I browsed through their interviews promoting the show and truly in most of them they sit like in the show :). Found only two so far whey they did not, but I think that the first one is from February where they in all likelihood didn’t have that agreed routine yet and the second is Graham Norton Show where they weren’t probably allowed to choose :).