I slept DD on her side from 3/4 months (without a sleep positioner) for similar reasons much better sleep she just liked it that way (that and she had a flat spot on the other side of the head and it was the only way to keep her off it) The Paed told my cousin once the baby can lift there own head and turn it they are fine. But yeah, it's a personal decision. DD never rolled onto her tummy in that postiion until 6 months so I was never worried.