I'm not 100% sure but I think just the renewal is necessary. If you have a DHCP address with lease time left and reboot when the DHCP server is unavailable, you will keep using the same address. If you do an ipconfig /release and /renew while the dhcp server is unavailable, you will get a MS 169 bogus address. If you reboot while the DHCP server is available while you have a lease with time left but the admin has set new information (like a reservation) then the computer will take the new lease.

When I am doing server migrations I will update DHCP scopes for things like new DNS servers and if users have difficulties I will often tell them to reboot. It's easier then walking them through the release/flushDNS/renew process.

Phil