I don't know if the OP asked this question specifically for post op MtFs but for FtMs it's kind of impossible to not tell a partner lol. I think it's absolutely crucial for a relationship to have complete honesty and I know I would tell a partner if I was transitioning or even if I was post-op.
Personally, I've always regarded transition as a life long kind of thing.. and in a sense you have to abandon traditional ideas of what a female bodied person is (XX, basically, with no genetic sex abnormalities) because as a MtF that's never going to happen. In terms of bodies tho of course, a post op MtF is very lucky that they can achieve what a born female would take for granted.. I just think that transitioning would be such an integral part of the person you are today that you would not be able to not tell a partner. It made you who you are.. and if a potential partner was so offended by the idea how could you accept them when essentially, you are in terms of body, a transsexual? it seems hypocritical to me. Then again I'd want to be involved with the trans community and be able to support other pre op or non op transgendered people without having to worry about 'secrets'.
Although of course a lot of trans people want to leave behind their trans identity when they fully transition (I may be talking solely about those who can achieve a full transition), so everyone's different.