I am really torn on this issue. Each morning I pass a group of four blue-haired beauties who are always walking shoulder to shoulder, blocking the entire path, even though signs all over the place clearly say you are supposed to stay to the right. It is a game each morning for me to pass them, and I never say 'on your left' because they are in the freaking way and 'on your left' isn't supposed to be a warning to get out the the way, just a courtesy to let people know you are passing. I have entertained using, "Get the f**k out of the way," but I haven't devolved to that point yet.
Contrasting this with others, who are on the right like they are supposed to be, I typically warn, but not always. Since I see the same people all the time, I know most of them are jamming to something spewing out of an iPod and don't hear a thing I do or say anyway.