Best practices are patterns that have proven themselves over and over again as a way to achieve a high quality of your solutions, and it’s completely irrelevant who proposed them.
I do know 110% that Logs and OS should be on different drives!
As for SharePoint I am really not sure. As Brian said, it doesn’t really grow, so it doesn’t endanger the stability of the system by risking the C:\ drive to get full.
I am really curious to see other opinions!