pipe compound

That is why I mentioned wiggling a flare fitting that leaks.
If you put just a little pressure on them with the nut, and then wiggle the line it can help mate the surfaces if you have one that leaks.

Otherwise,
Teflon on NPT threads starting 1 thread back to be sure you don't end up with a peice of teflon in your needle seat or jets.

Nothing generally on flares.

Pipe dope on your gas and water pipes in your house. :-)