I have read--and it seems to be true, at least for me--that you should get the smallest size shorts you can squeeze your patootie into. The reason is that extra fabric can snag on things like the nose of the saddle, causing some very ungraceful and maybe even painful dismounts. For this reason, I generally find myself wearing a size smaller in bike shorts than in any other shorts. YMMV.
I never heard that you should wear new shorts around for a while before riding in them, though if it's your first time in your life wearing bike shorts, I guess you might want to get the feel of them before hitting the road.