Generally this is true, but when it is something that must load on boot up
then you cannot use it till you re-boot or if the kernel is a new version
then it does not come into effect until you re-boot. In the case of
VirtualBox and certainly VMWare parts are compiled against the kernel as
part of the installation process and if you watch the boot process (instead
of using the graphical screens) you will see that components for VirtualBox
are loaded upon boot. I know that for sure VMWare will not work until you
re-boot and think that it is the same for the first time installation of
VirtualBox.

According to the VB manual you can work on configuring VMs in VirtualBox but
cannot RUN them until the kernel module is loaded. It also suggests that if
the kernel is updated this will have to be repeated unless you are running
DKMS. This can be easily done on Ubuntu by installing the package, but it is
not universally available across all distros.

So generally there is no need to re-boot, but there are a few exceptions.
Ubuntu will continue to work and you can even use upgraded applications, but
sometimes you are running the old package and not the upgrade until you
logout or in some cases even re-boot. In some applications such as Firefox
you need to re-start the application to take advantage of newly installed
parts. There is no universal rule such as you never need to re-boot.
However, you don't get nagged to death as you do in some OSes.