Whatever you do, make sure you know what you're going to say before you key the mic. (also, people down care how high/low the clouds are or the exact direction the winds are coming from..ie light winds out of the west should suffice.)
At my shop the PM is supposed to make all announcements, IE, seat belt in cruise, top of descent goodbye, etc. If the CA is flying, that means the FO is up. I've flown with captains to don't trust anyone to do anything, so I don't bother, I also fly with senior guys who just don't care, so after a leg or two, I ask if they want me to do it...more often than not they see it as one less thing to do.