Bec I think now that she is nearly 6 months, there is a fair bit less risk for her sleeping on her belly... I have known many babies that have done it...
As Ivana suggested, just make sure that you are able to keep an eye on her, and also make sure that she is on a firm flat mattress, nothing too soft and squishy - as I think its easier for them to suffocate on softer surfaces if they are face down.
I think its great that there is a position that she is really comfortable to go to sleep in easily without fussing and crying...