Irssi iirc used to default your username to your system username, so noobs would leak their given name by accident. After seeing that I changed my username in Linux to always be the most common username
I was talking more about you using HexChat with your preferred username "FooBar", but then when on Tor you want to be "SpamEggs". If you launch HexChat through oniux and it reads your config file, you might hit the login button before changing your name from FooBar to SpamEggs.