I do recommend encrypting your wallet, but I have another guide to cover that.

I'd be careful leaving any trace of your passphrase on the Raspberry Pi.  Instead, it's better to have a watchdog process to inform you if the `qtumd` process goes down or if the `qtumd` PID changes (so that you can manually bring it up again, then securely unlock it).  This topic is a bit more advanced though... but you have the right idea.