Well, using the standard client, when you make a new receiving address or when the wallet makes a new automatic change address you'd want to save that. I think the standard wallet keeps a pool of 100 reserve addresses by default (https://en.bitcoin.it/wiki/Key_pool) that gets saved as part of the backup, so you only need to do backups when you are about to use up the pool. In the past, many people lost BTC when they restored a wallet from an old backup that did not contain  newer receiving addresses that were created after the backup was made.

Of course if you don't use your wallet or your only use is for coins to keep coming in to the same address(es) then you don't need to keep making backups.