Here is a good KB Article on this subject - http://support.microsoft.com/default.aspx/kb/926172 and http://technet.microsoft.com/en-us/library/cc732351.aspx
Actually it's not a requirement and there are some issues that can come up around implementing it this way. Both articles you have referenced both state that it is not the Microsoft recommendation to do so.
the article describes that the dhcp-server should not be in another network and ip-helper shouldnot be used. on what machine the dhcp-service is located does no matter.