DADT still applies to trans people, so you wouldn't be able to tell them while signing up or technically even while in the service. But, the Air Force seems to be the most relaxed out of all the military divisions, and probably the least likely to make a big deal of it if a F soldier starts getting a deep voice and has a 5 o'clock shadow.
My brother-in-law is in the Air Force, loves it, and he can get away with a lot of stuff. I think the how lax it is can also depend on what type of work you're doing; my brother-in-law just does computer work so that's really relaxed. I am not sure how the ROTC would work with this stuff, I am not too familiar with their program, but you'd definitely need to consider being closeted for awhile if you sign up.