I tithe only when they ask, Molly. However, there might be some subtle game play I'm not aware of (for instance, the more "filled up" on tithing you are, do you get more xp/energy/mood?).
You don't have to tithe to get the Big Love. But if you want to get the Ruminate, Reflect, etc (which give you big rewards, the key to Icons) then tithe when they ask you. And, you can get the Rum, Ref. etc. multiple times between tithes.