I have to say, after puting some more thought into it, I think I'd have to agree with the few guys who are saying they wouldn't tell. I absolutely refuse to tell anyone my birth name now, since it's not who I am anymore, and never really was me to begin with.

I think if I passed 100% in all areas, and there was no chance of me being outed, I'd probably keep my unhappy past to myself.

I would probably still tell.  Some of the things I've been involved in in the past have been extremely gendered (for instance, I was very active in Girl Scouts growing up), and I don't want to have to alter details about my past, especially not to someone with whom I'm in a long-term relationship. 

i think I'll only be telling someone who sees that area and more than once. not for one night stands or a few week dating thing but as people have said, for a relationship there needs to be honesty, it'll be impossible hiding that or forgetting that part of my life, mostly since family still calls me girl pronouns and by my female name, there will always be photos of me around as a girl from birth to just a few months ago. there was sometimes little bits of pink on my clothes as a kid, on my bike, and being on girls teams. they'd wonder about that and I'm terrible at lying and I'm brutally honest so I'd tell a long term partner.

i think if, and only if, it looked and worked just like a cis guy's penis, and i was years past transition, i wouldn't tell. i had a somewhat gender-free childhood and would just say "when i was a kid" or "when i was little", and say my chest scars were from gynecomastia (which would make perfect sense with the rest of my story). but since that isn't going to happen, i plan on being open about my past. i want potential partners to know me fully and love me anyway