6 of one, 1/2 dozen of the other on all that stuff. If you ever plan on replacing the drop pipes you're gonna have to either cut off or unthread the stuff anyway.
For your question on the union, no you don't need teflon tape on the union nut itself. On the connecting ends however, you do need teflon tape on whatever is threading into the union.
Bit of advice - get enough stuff to replace the risers as well. If you're having problems with leaks, and patching the stuff already, if you unthread something, you're likely to have at least one problem with old threads.
Remember, CPVC needs to be supported every 4 feet.