recompliing c-language functions with new releases of postgres

I have several c-language extensions to postgres that I
distribute.  I generally have to recompile my code
with a new release of postgres, although sometimes it
appears that it functions properly (passes my tests at least)
without a recompile.  Since my users are all on various
releases of postgres, from 7.4.x to 8.1.x, I would like
to minimize the number of versions of my code that I
need to maintain and distribute.

Presumably, the only reason I would HAVE TO recompile
is when some header file changes.  Is there any guarantee
that header files DO NOT change, for example from
7.4.5 to 7.4.8 or even 7.4.12?  Can I assume that header
file changes only occur between major pg changes, such as
7.3 to 7.4, or 8.0 to 8.1?

TJ