I find that with longer lenses, I can be more covert.
When you stand there with the camera at your face, people tend to pose for you because they expect you to be there taking photos of, well, people. You don't have to necessarily use a longer lens like the 70-200, but you need to be at an angle that is not so obvious to the people. Then wait, wait, wait for your moment and snap away.
Here's a photo of a groom. It was late into the reception and he was taking a break outside smoking a cigar. I had the 70-200 mounted on a second body and actually was to the side just taking a breather when he came out. I just lifted my camera and lens, and took a few snaps undetected.