Is it a bit obscene if you wear partywear sarees a couple of inches below your navel ie a little low-waist?

It is definitely not obscene to wear a saree, party wear or otherwise, below the waist. Many women today consider it feminine and fashionable to do so. However, if you were modestly-raised, you might want to try wearing other clothing items, like jeans or skirts, low-waisted first, until you are comfortable with the idea.