my last two girls have been tummy sleepers since they could roll. With no2 she had a grow/sleep bag as i found she was always moving in her sleep and blankets wouldn't stay.. the sleeping bag slowed down her movement and annoyed her when she was crawling... but the benefits of her being warm outweighed the thoughts of her freezing during the night. She is also like her dad and loves sleeping with a blanket touching her face or over her head...
There are more "preventable" risks than just sleep position and blankets, so i wouldnt be too worried!!