From Bitcoin Wiki:

The wallet file contains the private keys for change addresses, and they can receive and send coins normally. However, the GUI in the default client does not display them in the address book, therefore a recommendation is to backup wallet.dat every 50 transactions.

PHS uses the same mechanic. So it's good idea to backup your wallet after every 50 outgoing transaction. Incoming and stake transaction doesn't count. They use no change addresses.