That would be a decision that you would have to make, but here's a couple of things to think about. The purpose of tithing isn't about being a witness. When the bible talks about giving, it says that it is better to give in secret because the purpose of giving is about your intent. Also, the money that is tithed is to help further the kingdom. The money tithed will usually go towards your pastor's salary, building expenses, and to help fund various ministries.
I don't know what purpose tithing in an MMOPRG would serve.