At least for travelling in the U. S., it doesn't seem to matter much if you haven't changed your name/gender markers--though that depends to a degree on the individual TSA agent you get. I've only travelled once while presenting as female, and my passport still has both my male name and M gender marker. My first TSA agent got really confused--she let me go with a "have a good day, Ms., I mean Mr., I mean Ms. etc." In Atlanta, I had a slight issue where a TSA agent took a long, long look at my passport and then at me and back to the passport repeatedly--much longer than normal for me. She then let me pass without any gendering, but it was akward.
Each time I was 'scanned' by someone after going through the metal detector, they chose a woman. I simply didn't speak while she tapped parts of my body. I assume they didn't realise I was trans* at all at that point. If your voice isn't passable, as mine isn't really, and you get a female TSA agent to scan you, not speaking might be the safest thing to do unless they have to ask you a specific question. Even though the TSA is supposed to be more accommodating to transgender travellers, you can't be sure individual agents will know what to do, so not drawing attention to yourself, if it can be avoided, is the safest option overall.
