I regularly use the Software Updater to keep 16.04 LTS up to date. Today when I opened the Software Center, there was one OS update available that "Improves performance, stability and security improvements". It also shows "bind9-host" and "1:9.10.3.dfsg.P4-8ubuntu1". Should I also install this OS update?
Yes. Whenever the update manager comes up with an update, it is a release-wide update (everybody's machine gets it). These small updates keep 16.04 up-to-date and error free between major and minor releases.
Some updates may not be "checked" be default because they are not for the core of Ubuntu. They may be for a third-party application. In 99% of cases, you will want to install all updates that show up from the update manager if its settings are default.