I am on an updated Ubuntu 12.04 and I've found a bug in the tag feature of Nautilus. I'd like to file this as a bug report, but I'm confused to where to post it: Ubuntu's bug tracker in Launchpad or GNOME's Nautilus bugzilla?
I think it would be important to note that before posting the bug upstream, one must be sure of this:
Developers upstream aren't too keen on debugging stuff that has been altered by a third party. Similar to asking for support at the factory while you bought a product from a reseller/retailer which has repackaged it.
Therefore, I would recommend posting it on the Ubuntu bug tracker only first in case you're in doubt. In case this is an upstream bug, Ubuntu developers can decide on (back)porting the fix in the version in Ubuntu like all regular bug fixes.
If, some time later, you're able to find evidence that other distributions (other than Debian-derivatives) suffer from this as well, or a Ubuntu developer can point out it is not a packaging issue, then you can open an upstream bug report including a link to the Launchpad bug report. Launchpad is able to track several major external bug reporting systems, so the information can be linked, rather than completely independent.
Either way, a bug report on Ubuntu's bug tracker might help indirectly in decision making for the Ubuntu developers. Suppose case tons of "hot" bug reports for application X come in after Ubuntu release Y to include this, then it's a good way of evaluating the decision on replacing X as a default application or for future decisions.
My conclusion to this is: start by filing a bug on Ubuntu's bug tracker and only post upstream if you have evidence to support it's not Debian/Ubuntu-specific.
You could report it in both places. On Launchpad indicate that a GNOME bugzilla report exists for the bug. Then the Launchpad bug will monitor the GNOME bug so that if GNOME addresses it it will get incorporated by Ubuntu.
Reporting in both places helps ensure that both ubuntu and GNOME developers know about the bug and hopefully whichever team can best address it will take it on.