I have read a bunch about this and I see the same advice quite often. Start as soon as possible.
I didn't see this mentioned in this thread, but I will parrot some of the reasons I have heard. They may or may not be accurate. So feel free to correct me if I'm wrong.
It can be pretty uncomfortable trying RLE when the afternoon comes around and your 5 o clock shadow is starting to show thru your make-up. For somebody who is a little more self-conscious like I am, this is important to my comfort, which effects my confidence. And you know we have to be confident to pass well.
I have also read that after you start HRT, you become a little more sensitive to the pain of electrolysis. I'm not SURE that this is true, but it sounded good to me.
. . .On the flip side, after you start HRT, the hair gets lighter and easier to remove. So it might be cheaper to wait till after HRT has begun because you will probably not need as many visits to remove all the hair that you want gone for good.