One also sees the common advice to include the paired header (ie, foo.c should include foo.h) as the first #include to verify that foo.h is dependency-complete.

The problem with dependency-complete headers is that once a .c file relies on that header for a symbol, if the header is changed to no longer require the symbol, all those .c files have to have their #includes fixed. This can take a lot of time to iterate through in a projects that require hours to compile.