As long as you read all the security precautions they list here and use an offline version of the number generator as opposed to generating addresses live on the website (or any website really) it should be OK.

You then get the "blanks"... So you can generate your addresses on an offline machine using a copy of the address generator, print the details of your addresses onto the blanks using a "dumb" printer... and for extra bonus points you can then add the nice hologram tamper stickers and put them in nice waterproof storage bags as they recommend and you should be good to go.