What if you cannot find your ancestors on the 1870 census?

I can find my great-great grandparents on the 1880 census in Greene County, GA but have never been able to find them on the 1870 census (have looked for years). However, I recently came across the death certificate of my great-great uncle that places the family in Greene County prior to 1870 (great-great uncle was born abt 1866) and the death certificate of a great-great aunt that places them in White Plains, Greene County, GA abt 1871 (great-great aunt was born about 1871).

I know it's not safe to assume anything but would it be safe to assume that my great grandparents are in Greene County, GA in 1870 based on info that places them there just before 1870 and after 1870?