c-language function .h file changes between (major) versions

I've been developing c-language functions in 7.4.5 and 7.4.8.
I had not encountered a need to recompile, because of pg .h file,
or other differences. I expected a need to do so when upgrading
to 8.0.3. But I forgot to do so and it worked!! I figured I
lucked out, a suspicion which was confirmed when my functions
failed with 8.1beta1, but only some functions. A recompile
restored correct operation under 8.1beta1.

So, my question is: Is there some hard and fast rule about when
one needs to recompile c-language functions from one pg version to
another? Always for a major version change? minor? I suppose
the wise thing is to recompile with ANY pg version change.

Thanks,
TJ