There is wallet software that uses "deterministic wallets" (BIP 0032) that use keys that are derived from a single seed. For example, Armory. Those you only need to back up once.
But for example Bitcoin-Qt (and Multibit AFAIK) does not, so you need to back up regularly if you use the wallet actively as new keys are added to the wallet all the time.
By the way, with the QT clients, if I've encrypted my wallet with a password and I "Backup Wallet", is the backed up wallet encrypted also?