If the bug tracker supports voting, some projects use voting. It
looks like google code doesn't.
A lot of open source projects don't do actual voting though. The core
team or developer sets the priority based off of their own needs and
is influenced by the noise on the mailing list as to what to tackle
next. And of course, feature requests that have patches submitted
with them tend to bubble up because the author doesn't have to do much
work to get the feature supported.
-Mat