A few years ago babies were commonly put on their tummies to sleep, but a new movement is about primarily with reference to SIDS (sudden infant death syndrome) or COT Death that seems to preach that babies are safer being put to sleep on their backs.
Hence the entire movement of pillows that prop baby to prevent babies rolling over from their backs to their tummies.
I am not much of an alarmist and tend to take most things with a pinch of salt, and a dash of reality-check, but I personally have always put my kids to sleep on their backs - and as infants used a prop pillow to keep them on their backs.
That has been my experience.