Upgrading within a major version (5.x to 5.y, 6.x to 6.y, 7.x to 7.y, etc) normally does not require any rebuilding of installed ports. FreeBSD devs try very hard to maintain compatibility within a major release. Check /usr/src/UPDATING for details.
Upgrading across major versions (5.x to 6.y, 6.x to 7.x, etc) requires rebuilding all installed ports. One can usually put off the rebuild for awhile by installing the compat5x and/or compat6x ports (the one for the version you upgraded from). However, the long-term, most stable solution, is to rebuild all installed ports.