I think there is more depth to this question than is apparent on the surface. At what point do we share information with people that may make them uncomfortable. D&J made some great points about the need for discretion. I can see where sharing this information too early could cause a person to prematurely abandon a relationship that could turn out to be very beneficial for both parties. I think DarkMist makes a great point, too. I think it would be very unfair to, for example, marry someone without them knowing that you like this activity. I have read many posts on these forums from women who have found that their husbands were involved in activities that they considered "deviant." The sense of hurt and betrayal permeated the posts.
This is an area where we will all be seeking balance long after this post is history. I think the guiding principle needs to be that it is unfair to let someone make a major decision without access to information that could impact their decision. The woman is eventually going to find out that you crossdress, and I think it's better for her to know it upfront than after she's in a committed relationship. If she is accepting of the behavior, she will wonder why you didn't trust her enough to share it with her. If she can't accept it, she will be angry for hiding it for her. Either way, she will wonder what else you were hiding from her.
A rule of public relationships is that when upsetting news has to come out, it's always better for them to hear it from you. I probably wouldn't tell someone I was seeing casually, but as DarkMist says I think it would be unfair to spring something like this on someone late in the game. I think this issue calls for careful consideration and balance applied to the specific situation.
Another issue underlying this question is the topic of compatibility. If someone can't accept this part of you, which is obviously a big part of who you are, then you need to accept that she isn't the right person for you. You don't want to be married/committed to someone and living in fear of them finding out about this. You deserve to be with someone you can feel free to be your real self with. If you have to hide a part of who you are, then the relationship isn't the right one for you. You deserve someone who loves you for you. Therefore, if she can't handle it, it's better to find it out before the relationship goes too far.
I want to thank you for bringing this issue up on the forums. It is a very delicate issue, and I'm sure this won't be the last post about "When do I tell him/her.....?"