Is it necessary to upgrade to this new version right away, since LTS distro's are supported for 5 years?
It is not "necessary" and you're not "required" to upgrade if you don't want to. Generally, it's recommended that you do so, but it doesn't need to be "right away", as in immediately.
You can wait for the first point release of 18.04.1, which comes out roughly one month from initial release. At that time, the system will show a message that a new release is available, in which case, the command
sudo do-release-upgrade will find and apply the update.
However, if you choose to upgrade before then, you can do so with the
sudo do-release-upgrade -d (notice the extra
You should take a look at the documentation, as it lists the known issues and other things you should probably be aware of before you choose to upgrade.
 By "necessary", I mean in the sense that your system won't stop working if you choose not to upgrade.
 By "required", I mean you're not obligated/forced to do so.