First, I came out early myself (to my immediate team of six) to give them a chance to acclimate. Will come out to the rest of the department a few weeks before the actual transition (date TBD).
As for past tense, everything I've read and heard indicates that you should ask people to refer to you as your female self, even in past tense. Technically, you have ALWAYS been her, even if your coworkers didn't know. Also, this helps reinforce the idea that you are female, helps avoid confusion, and makes it easier for new employees to acclimate. It takes time, but they'll get the hang of it. A good friend of mine that I've known since high school (FtM) transitioned a few years ago. It was tough at first, but I can't even remember his female self and when I talk about school or one of the many sci-fi cons (Star Trek, Dr. Who, Blake's 7) we attended together, I always think of "him" being there and not "her."
By the way, one of the strangest questions I received after coming out was "What do you plan to wear?" Not sure what they were afraid of (maybe they didn't want to see my legs but tough on that) but I told them I would dress appropriately for work of course, and told them that my style would be more conservative than flashy.
Jane