My DD has been a tummy sleeper since about 6 weeks. She couldn't go to sleep on her back and sleeps beautifully on her front. She has good neck control, and many generations before us (including me!) slept on tummies. I think I have reduced SIDS risk factors everywhere else (linen, positioning, not too hot, no bumpers/toys, no smokers, good birth weight, breast fed etc etc) so am happy to let her. Depends on your comfort level with it....