Yes. We strongly recommend sticking to just letters, numbers and underscore. Right now, we reserve the characters # ^ ~ |. # is used for firewall differentiation (see above). We want to save ^ | and ~ for other problems which might come up. Also, don’t put spaces in your username; please use some character like “_” instead. Finally, please note that usernames are case sensitive, so “Dave” and “dave” and “dAVE” are all different usernames.