You have to consider though, that the advice to put babies on their backs to sleep is based on recent research into cot death. The evidence is there that it's helped to reduce the number of cot deaths significantly.
Once baby can roll over, they say it's OK to leave them in whatever position they move into but still to put them down on their backs when you first put them down in the cot.
You have to do what you think is right at the end of the day.