All for show - I like your idea of offering up a few concessions as a bargaining chip. An annual meeting could announce birthdays are ok, blood is a conscience decision, and 10% of our income is expected by Jehovah in order to receive Holy Spirit and blessings.
They probably wouldn't send a bill per se or ask to see your tax return, but would harp on it and question your love for Jehovah if you were holding back your "first fruits". Maybe even referring to it as a secret sin. Elders could ask if you are giving your "share" when they go on shepherding calls. They could guilt trip publishers into thinking they are holding back their own blessings by not giving Jehovah what is rightfully his, even implying they are stealing from God.
Wow, it's sad how diabolical you can think when you've been raised in a cult... But doesn't that sound just like something they would do?