If you are sure that the bug you want to report is due to a problem in a software, not related
to the fact that the software is used on a Gentoo machine, then you are encouraged to report
the bug to the software author(s): rarely Gentoo developers have the required knowledge to fix
the software code. Moreover, it's a good open-source behavior that the bug reporter get directly
in touch with the code maintainer to improve the feedback quality.
If you are unsure whether the bug is due to the software or is Gentoo-specific, feel free to file
a bug, but consider that if the bug is not Gentoo-specific it could be closed with resolution
"UPSTREAM", maybe with a suggestion on how to report the bug to the software authors.
If you think that Gentoo developers should take some actions with respect to some software bug,
for instance applying a patch to an ebuild, feel free to file the bug explaining the severity
of the bug and suggesting possible actions.