Well, I always include stdio.h which includes stddef.h that defines NULL as (void *)0.
In my experience, hardly any source files require studio.h
stddef.h on the other hand is required by most to get size_t
I talked about my personal experience, but yeah, as you said, stddef.h is often required (and yes, often I do not need stdio.h, stddef.h is what I need) which defines NULL, which was my point. If it is often required, then it does not matter whether you have to include a header file or not, IMO.