I would say that you shouldn't cut taxes to favor particular social outcomes. That would be against the liberty of the citizens. How would you feel if there was a 'Christian tax cut'?
Instead, we should be reducing the number of special groups that get preferential treatment by US tax law. Close the loopholes, and ensure that everyone is required to pay their fair share.
In such a way, you'd remove some advantages of having children (child tax credits).
On the other end of the spectrum, I wouldn't worry about birth rates. The birth rate in the US is just above replacement, so it wouldn't do us much good to go into the negative ala Europe and Japan. We need to be population neutral or slightly positive...Not dying off. Unless, that is, if you want all the undeveloped nations to be the growing ones in the world 
Back from the dead, I'm afraid.
