Yeah, I usually leave them there for a few days after I install an update to make sure everything's working alright before deleting them. But they're safe to delete.
Be aware that you won't be able to uninstall them using the entries in Add/Remove Programs, so you might want to edit the registry to remove those entries as well. Look for the respective keys under: HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Uninstall
There's a shareware tool called Update Cleanup that makes it easy to delete these $NtUninstallKB folders and clean up the related Registry entries at the same time. Although it is not free, you are given a lengthy free trial which is more than enough time for you to clean up these folders.