I am sure that the troops recieve instructions about not revealing information in letters but I do know that in WWII the soldiers letters were routinely monitored. My grandma has letters that my grandpa sent with black marked through things that the Army blocked out.
I do think that it's a slippery slope. I am all for national security but I think if the government starts taking liberties with our rights, it leads to a place we don't want to go. I don't want to live in a police state where there is no privacy in the name of security.