An option to tax temples is totally fine. And would be a very, very fun political tool.
But a universal taxation of temples will undermine an already struggling aspect of the game, religion.
Yes and no.
Lords would love to have religions, and especialy rich ones build there temples in his or her region knowing it will bring him extra income.
The more temples the better.
but still an option to tax them or even an option on how harsh to tax would indeed give more dimension, fun and flexibility, like where used with BM.