Personally I have found it to be less of a problem than I imagined it would be, even pre-op. When I was pre-op, there were a couple of times where I was reckless enough to get into bed with a guy without even telling him! I just kept everything hidden well... But when it came to actually telling a potential guy in two instances, in the end it became a non issue, but it was largely because I disclosed AFTER he got to know and like me, and not from the off. I wouldn't recommend telling straight away as it will shape his perception of you and possibly not allow him to get to know you in the way you would like. However, with anyone open minded and accepting enough, the general perception I seem to get is that it is merely a side issue as it relates to my past, and does not affect the way they feel about me, because they were interested in me as a person. For some, it may affect their perception throughout a relationship as they process what your past means, and for some, a woman who in the minds of some "used to be a man" may not be somebody they can see themselves with - but those aren't the sorts of people you'd want to be with in any case.
In short, trans status can be used as a good filter.
However, you don't have to feel sorry for your trans status or feel you are tricking anyone. I can appreciate that pre-op you may be conscious of somebody being shocked at the fact that you don't have the body they may expect but in my view, it is a matter of a potential partner being interested in you, being attracted to your soul, meaning your physical past or even present state is not something that everyone is entitled to know straight away. The more confident and happy you can be in being assured that somebody can be attracted to you as a person and that your transition is simply an element of you (or being confident and happy in being openly trans, but then "telling" wouldn't be such an issue, so I don't think this applies in this instance) the more you will convince people to like you and accept your trans status. I wouldn't worry about whether you 100% pass or not either, it should be purely about the interaction between you and your potential partner.
I compare the ideal attitude to dating to an amusing sketch by Eddie Izzard when he mentions shopping for women's clothes. He says that you can't go in and ask the shop clerk in a small apologetic voice "er...where erm... can I try on this....dress?" you have to walk up to him and say "I AM trying on this dress, now hurry up and show me the changing rooms!" (it's funnier when he tells it!). But in essence, the message is that you should not need to be apologetic about disclosing or self conscious, it is simply a matter to be disclosed at some time and your partner or potential partner should take it on board or leave. The more you act as if your being trans is something that ought to be accepted rather than being self conscious, the more they probably will actually accept you.