70% of my business is ministers. This is very common in certain denominations. I have been through dozens of audits where this issue came up. While many call them tithes, they are essentially district dues or credential fees (depending on the organization and where they are paid). If you have the district or general bylaws of the organization that states it is required, you will not lose an audit by claiming this as SCH C expense.
Be careful of Clergy tax guides. There are so many different denominations and structures, it can be as different as preparing a return for a farmer and a pastor when you are looking at two ministers from radically different denomination structures. Many of the guides are written as a "catch all" that generalizes structures like Methodist, Southern Baptist, Lutheran. Some structures are radically different and need to be examined.