I use Ancestry and similar sites to acess documentation that used to require a trip to the the archives or courthouse or library. It allows a breadth of research that would otherwise be time and cost prohibitive for me. But although I may have looked at Ancestry to read a census, for example, I cite the underlying primary source as Ancestry itself does (e.g., year, state, county, enumeration district, page, line) so that others who do not have access to Ancestry could go to another publicly available source, be it online or bricks and mortar, to find the document themselves.
Many more sources are coming online everyday. People doing research in Missouri, for instance, will have found that the State of Missouri is putting their death records online, with a link to the pdf of the actual death certificate. In some cases for research I did years ago I look to see if the souce is online and link to it, but honestly I'm not sure if that's a good use of my time to go back and do that for eveything if I have cited the underlying source.
