Actually if you want to be 100% sure, you need to do/take few more precautions:

1. You need to be offline

2. And again - being offline while you are creating and saving your wallet.

3. Without going online, low format all computer drives after the wallet creation.

The easiest way to do it, is to make a full mirror of your HDD/SSD before going offline and wallet creation, then format all and then restore the mirror.