Not 100%

The ProGenealogists blog did a very nice thing in getting permission from author William B. Saxbe, Jr., to reprint his article from the March 1999 National Genealogical Society Quarterly, "Nineteenth-Century Death Records: How Dependable Are They?"

Read the whole thing, but basically Saxbe carefully compared three different records of deaths in Champaign County, Ohio, between 1 June 1879 and 31 May 1880: a county death register, the US census mortality schedule for the county, and obituaries published in the three county-seat newspapers. Conclusion: "No more than 35 percent of the known deaths produced obituaries, only 56 percent appeared in the county death registrations, and only 84 percent were picked up by that year’s mortality census." And almost half of the known deaths appeared in only one of the three sources.