My 2 cents is that anything that can be derived from source should be 
by default.

The extra cycles chewed during compilation aren't as important as 
consistency and correctness.  Using cvs checkin as a trigger might 
introduce a disconnect between developer working directories and the 
production repository.

I also think that rdoc should be run using the ruby and rdoc in the 
working directory to close the development loop, i.e.

rdoc: ruby
     ./ruby -I lib bin/rdoc --all --op ./rdoc *.c

And then just installing from working dir ./rdoc to <sysdir> during 
install phase.  The reason for this is that you wouldn't want a process 
which might fail running during install.  Better that it die before 
anything in production has been touched.

I guess my point is that by default you should be encouraged to update 
and test your rdoc comments before either checkin or install.

-J