I have just reread your very good article again, to be sure it doesn't cover this peculiarity — which is not about a census as such, but about the indexes which accompany each census.
If I correctly understand the soundex system, any query for "McCormick" should also find all the "McCormack" entries that match the other search parameters (as long as I do not mark "exact.") Is that true?
There is a belief in my husband's family that his grandfather was born McCormack, but that the name was changed to McCormick sometime before he married. Searches made on McCormick don't seem to provide McCormack alternatives.
Is it necessary for us to search under both spellings?
No, we do not have ancestry at this time. Mostly we are searching the pilot site at familysearch.org plus the 1850 census and 1930 census shown on footnote. There are also some census "snippets" that turn up which hold some of our information.
I don't use those sites for census, but if there is an option for "exact spelling", do not check that and it should return all hits within that soundex range I would think. I did a quick search on the familysearch.org in the census records for "McCormick" and did not check the exact spelling box and received hits for McCormick, McCormack, McCormic, McCormac, MacCormac and a few others. If you are not checking exact spelling and you are looking in a specific area, it could just be that there were no McCormacks there, but you could do a separate search for McCormack just to be sure.
I have thought that I was leaving the "exact" boxes unchecked, but I will be more careful in the future.
I guess I've being a bit "lazy;" it's not that much trouble to run a second search on the alternate spelling in those areas family tradition says hold that spelling.