Nappy, I think there are more risks to a natural 3rd stage if your body has had drugs. The benefits of it are also greatly reduced as there has already been interference with the hormones etc.
If I had had medical intervention I'd probably continue with it into 3rd stage tbh.
There is some research that suggests that there is more bleeding at the time if the birth with a natural 3rd stage as the blood vessels don't clamp shut, but it can be argued (and is) that the additional bloodloss with a natural 3rd stage is necessary and if not coming away then will come away over the next six weeks in heavier lochia.
Natural third stages are more likely to be the cause of pph when the mws are not experienced in them and aren't sure which interventions are fine, or if they feel anxious about the time taken and try and assist with a tug.