I wonder what is best to use
- case sensitive usernames
- case insensitive usernames.

Both are possible.
If I use MySQL with charset utf8, there will be default collate utf8_general_ci,
in what 'ci' stands for Case Insensitive.

But I can make table be COLLATE utf8_bin,
which will make the contents of th table or a column be Case Sensitive.

By default is case insensitive is used in database.
So 'admin' will be treated as equal to 'Admin'.

I wonder if there is any good reason to use the case sensitive option?