I will take it one step further. Don't download anything you don't need just because you are curious to test it out or see what it does. "Curiosity killed the cat", remember that. Stick with the stuff you know and you need. When you determine that a file is dangerous based on what it did to your system, it might already be too late and something awful already happened. Keep it far away from you if you don't know what it is.

That goes for desktop computers as well. Don't access Bitcointalk from internet cafés because you can't know what the owners have installed on those computers or what the person who used it before you did. Using a friend's PC/phone can also be dangerous. Don't use unsecure WIFI networks. The friend doesn't have to be malicious and want to hack you, but they could be infected with some malware themselves. 

That message needs to be stored safely as well. If you get your device hacked and someone steals your digital proof, the other person will also gain the ability to sign a message from the same address and ultimately prove they have access to the private key.