I find it utterly important to tax religions, because too many people have begun using them as banks.
Which is VERY medievally correct. Religions should be heavily advantaged (or have the possibility of a heavy advantage) in the financial markets, if we want to be even remotely Medieval.
It might even make it easier. The Lord receives funds he will replenish, which adjusts his gold balance, making it easier to make large withdrawals for one person.
Galvez' point is correct. Taxing guildhouses makes running mass money transfers easier. My character in Terran has hundreds of gold in surplus with every guild he is in (which is quite a few). Why? Because he taxes their treasuries, then replenishes them. As such, those guilds never actually gain much and never have a large sum of money on hand, but Hireshmont racks up surpluses he can use to shift around moneys however he wants. And I do this very aggressively, allowing me to move money very easily.
The zero-tax on temples means the only way for me to get a big surplus for religions is to actually give them cash they get to keep. Meaning I can never get net positive transactions (except for monthly account balance changes, but those are practically meaningless in most cases). Taxing temples will simultaneously raise major new overhead, decrease the possibility of large religions and temples prospering, and make financial tricks much easier for clever lords.