I'm US citizen and have traveled extensively to Shenghen over the last ten years, and only once (in Germany) was asked about the return tickets. I didn't have them with me in hand (only on computer), and was let in without showing them. The remaining dozen times nobody asked for anything.
This also depends on the country you're entering the Shenghen area. Netherlands and Germany, for example, are those who ask most questions (purpose of trip, where you stay, how long), while in Italy or Spain nobody has bothered asking anything, and sometime they even didn't stamp my passport.