In general terms, I would say as follows:
1. All user names must use only basic latin characters (no accents)
2. All user names should start with a letter
3. Use of punctuation and special symbols are discouraged, especially if it might cause confusion between two active members
3a. We don't allow user names to have multiple special symbols in a row
So if we're given the choice between a username with punctuation and without, we would choose without. And if at all possible, it would better for people to have unique usernames that won't be easily confounded with another member based only on the inclusion of a symbol. (The memberbase isn't so huge that it's impossible to find unique and interesting names.)
I'll see if I can get these points codified and added to the opening post in order to reduce future confusion.