Capitalization of the username doesn't matter, but it does for the password.
ok that was all i wanted to know 
Well hold on a second. I just noticed something funny after testing that for you..
When you change the capitalization of your name, the client doesn't load your custom playerslot.txt file.
So I guess in that regard, it DOES matter. If you want to write it a different way, you'd have to rename that folder to reflect the same capitalization.
o that's weird
When you log in does your user name need to always be how you wrote it out when you first made the account? Don't really need to know but its been bugging me for a few seconds now