If they have a gasket inside, you don't need any tape or compound. If it's old and it leaks, the gasket may need replacing. If it's new and it's leaking, you're not tight enough, or way overtight and ruined the gasket.
I just replaced a leaking flex line for a water heater that was leaking. The handyman who installed it I have respect for, but he did put a ton of pipe dope around the connection, I don't think it helped.
I did not put any tape or dope on the threads and it is doing fine, thanks.