Britney. OF COURSE NOT! What kind of rude assumption.
Gaga lip-synched ONCE at the performance of Telephone at the Monster Ball in the O2 back in February. That was the only time I can remember.
I definitely know Beyonce has lip-synched before. Hell, there was a performance of hers where she didn't even wear a mic.
Madonna. Most obvious one is VMA's 1990. She didn't have a mic either.
Katy. I don't think so, but it would be nice if she did.
Rihanna. I'm pretty sure she has.