[apaprocki]

To be fair here, NASA does say to avoid nesting headers. But then it follows up with this ridiculous statement:

  In extreme cases, where a large number of header files are 
  to be included in several different source files, it is 
  acceptable to put all common #includes in one include file. 

I would not trust anything else the author of that guide writes. EMBOSS says not to nest. The Atacama code suggests not nesting but then says:

  To avoid nested includes, use #define wrappers as follows:

The last pdf states:

  This prevents including the same item more than once when 
  a .c file includes several .h files (we rejected
  alternatives involving conditional compilation as too 
  complicated).

I guess it boils down to preference since the language allows you to do either, I just don't put a lot of stock in these style guides.