user4118: Is it obligatory to backup wallet.dat after each outgoing transfer?
I understand it is, but not after each incoming transfer.

user2306: Every 100 transactions, counting both sending and receiving. For receiving, you can treat multiple transactions to the same address as a single transaction.
This ensures that you don't lose any bitcoins if, for example, your hard drive crashes.
The option keypool in Bitcoin-Qt client adjusts this.

4118: <p>Is it obligatory to backup <code>wallet.dat</code> after each outgoing transfer?</p>

<p>I understand it is, but not after each incoming transfer.</p>
 2306: <p><a href="https://en.bitcoin.it/wiki/Key_pool" rel="nofollow">Every 100 transactions</a>, counting both sending and receiving. For receiving, you can treat multiple transactions to the same address as a single transaction.</p>

<p>This ensures that you don't lose any bitcoins if, for example, your hard drive crashes.</p>

<p>The option <code>keypool</code> in Bitcoin-Qt client adjusts this.</p>
