For the Desktop installers of the various 18.04 flavors, there is no need to go and manually activate Universe, Multiverse, or Restricted - they are enabled by default. This also applies to the Server installation when using the Alternate Installer.
(Note that the only exception to this is on the Live installers themselves for Ubuntu itself - they don't enable Universe by default but only on the Live sessions. Kubuntu, Lubuntu, etc. all have Universe components and have it activated, it seems)
However, on the latest 18.04.1 Subiquity-based Live Server Installer ISOs, there is a known bug where the ISOs for 18.04(.1) install but don't enable Universe or Multiverse repositories.
For those cases, you have to go back and enable Universe and Multiverse and Restricted; keep in mind that this is a bug in the installer and not typical as it's supposed to enable Universe and Multiverse.
(This has been fixed for future ISOs including the 18.04.2 (hopefully) and 18.10 ISOs that are going to be spun when 18.10 is actually released. (However, the 18.04.1 Server ISOs - but not the Alternate istnaller ISO - are known to be affected by this bug))
If you are using a preinstalled image on a VPS or such, they have a wide variety of configurations, so you may have to enable those repositories manually. This is outside the control of Ubuntu however, as those VPS images tend to have a few additional changes by VPS vendors.