No, that would be overkill. The GENERIC kernel always has 'compat' settings for previous major releases (and ports compiled under them), so they will continue to work under a compatibility layer. Once you arrive at your final OS, portupgrade -af is advisable. Moreover, doing a full portupgrade is only necessary when upgrading to the next major level (6 -> 7 -> 8), not the next minor level (6.0 -> 6.4, 7.0 -> 7.2)