Location is one of the most important things to determine, since so many records are location based. Tax and census records are clearly location based, but so are church registers and many wills. Always identify, to the greatest precision possible, exactly where an ancestor lived, and when.
When you know the location, one of the handiest things you can have is a contemporary map. Unfortunately old maps are not easy to find and usually do not show what you want to see. Probably the best source of Ontario maps are the county atlases. These large books were produced in the 1870's and 1880's for most – but not all – Ontario counties. Many were reprinted as a historical project in the 1970's and 1980's. They contain excellent township and community maps. They often have the name of the resident on the map but beware! – some of these atlases contain only the names of those who paid for the privilege. Libraries tend to have local ones. A number of county atlases are on line at In Search of Your Canadian Past: The Canadian County Atlas Project.
Other online sources of maps include:
Map of Ontario (1874) used for 1871 census
District Maps Of Canada West/ Upper Canada /Ontario - 1836 and 1845
Map companies such as Perly's (founded 1949) and Map Art (founded around 1980) produce annual versions of maps. A pre-1960 Perly atlas of Ontario would be a valuable item, if you can find one.
A gazetteer will provide the location of every named feature. There are a number of them, some more complete than others, and of course old ones will provide old names that are no longer in use. A good modern one is Supply and Services Canada – Canada Gazetteer Atlas – MacMillan (1980) ISBN: 0-7705-1873-7. An Ontario website is Ontario Locator
Natural Resoruces Canada's website Querying Geographic Names of Canada offers a search name of presnet and past names. For current Ontario locations, use the Ontario Ministry of Transportation's online road maps.