When you install a software or update and it asks you to reboot, it does this because the installation can't be completed until next reboot. If you installed a program, and don't want to use it immediately (for example because you want to install several other programs first), just say "do not reboot". I don't think there are many programs that reboot the computer without asking you first.
It is really a pain in the ass though that automatic updates agressively ask you to reboot (while most of the security updates they fix aren't of immediate danger). Just do as Mepher said.