This will probably throw an error if there are no packages to upgrade. You might wanna add the "-r" switch to xargs, so it doesn't run opkg upgrade if there are now packages to upgrade. And the field seperator switch -F shouldn't be necessary either, since it will split on the space preceding the dash anyway (or following the package name, depending on the way you look at it), so this should do:
opkg list-upgradable | awk '{print $1}' | xargs -r opkg upgrade
That probably depends on which packages specifically you upgraded. For some it might be enough to just restart the service in question. Other changes, such as binary firmware packages, are only effective after a reboot. So, as a general rule of thumb, a reboot might be a good idea.