But then if DR site goes down: "Loss of the mirror and the witness

Assume you have configured database mirroring with a witness. When the mirror is unavailable, the principal runs exposed. While the mirror is unavailable, if the witness is also lost, the principal becomes isolated and can’t service the clients. Even though the principal database is running, it is not available to the clients. If you attempt to connect to the database, you get the message “Database <dbname> is enabled for database mirroring, but neither the partner nor witness server instances are available: the database cannot be opened.”

http://technet.microsoft.com/en-us/library/cc917681.aspx#EIAA

So without a third site it depends, I think you should place the witness at the more reliable site.