Yes, deleting a package will not remove its dependencies.. perhaps you may want to install something in the future that needs them, this can save you bandwidth, disk space is cheaper than bandwidth.
You can use
pkg_info -t to see a list of installed packages that do not depend on other packages, which might make it easier to clean up the stuff you don't want.
Opera is closed source, the port probably grabs the Linux binary, if it's compiled statically.. there wouldn't be any dependencies, otherwise the only thing it would depend on would be the
fedora_base package.
Learn to be friends with
pkg_info(1) and
pkg_delete(1), they can help paint a nice picture of inter-package dependencies..
pkg_info -R gtk+2 for example would show which packages depend on GTK+2.
It's not a rule here I believe, but the colour red sure looks annoying on this background.