upstream Source patches and patch systems

[fabrice]

Hi,

We were having a discussion yesterday on IRC about the sense of adding a
patch system to a package that don't have one when modifying the
upstream source code.

My understanding of the general rules is:
- if it's a Debian package and modification has already been done, stick
to what the Debian maintainer is already doing. That mean use the
existing patch system, if any, or modify directly the source if the
Debian maintainer already did some modifications, but do not add a patch
system.
- if it's a Debian package and no previous modifications has been done,
modify directly the source, and do not introduce a patch system
- if it's an Ubuntu (-0ubuntuX) package, the same rule apply, with the
added rule that adding a patch system is preferred when no upstream
source modifications has been done previously

I just wanted to check that there is a general agreement or disagreement
with this approach, and check that I'm not messing up new contributors :-)

Thanks,
Fabrice