If a patch needs to replace a file that is currently in use by the system or an application, a reboot is needed. You can check this in the registry under the following key:
If there is a key named "PendingFileRenoameOperations" then a reboot is needed. If not, you don't need a reboot. You can install the patch through patch manager and check this registry key and you should check under Programs and Features if the patch was really installed. Just to be sure the tasks says indeed success but nothing really happened on the client...:-)
Yerp - all to do with the "PendingFileRename" by and large.
That said, it's usually a good idea to "ask / force" a reboot anyway. That way, you can at least be sure that the devices will at least reboot 1x month (and flush out their PendingFileRename queue).
I've seen especially *cough* sales *cough* people with laptops that haven't seen a reboot in 6+ months (and then - marvel oh marvel - were causing problems left right & center).