rebooting gains you nothing except not understanding what you are doing.
all u r doing in this process is replacing a single file that is presently running, so simply telling it to stop the old one and start the new one is all that is needed... u r not changing or modifying anything OS related to need a reboot.
just like u dont need to reboot when changing frequency...
In the same breath you can say simply browsing to System->Startup and clicking Restart (in the cgminer row) after downloading does the trick just as well. But in the grand scheme of things, there is nothing of any significance to loose by rebooting.
Just saying ....