Twenty years ago everyone laid their babies on their stomachs or sides. It was generally accepted that this was the least risky position to sleep on, and obviously everyone in my generation isn't dead.
The fact of the matter is, no one knows what causes sids, at least not definitively. They know what can increase the chances of it, but thus far there is no conclusive evidence of cause.
When my son was born, I was so bombarded with sids this and sids that and I was terrified all the time that if I made one wrong move he would die.
The point is, sids is a very real and very serious thing. But it's also a very rare occurance. It should be taken seriously, and you should be careful, take precautions, watch carefully. But every baby is different, some prefer to sleep on their stomachs, some on their backs. Some sleep on their sides (as mine always has).
If it's daytime and you're keeping a watchful eye, by all means do what you feel is best. The best advice I can think of to give any new mother is relax, enjoy, trust your gut. You love your baby, you will know what's best for her, because she is unique.
Take care. :)