There are a lot of threads out here about that sort of question, and there are different answers depending on exact situation.
I would recommend that you use at least 1 TC to run level 20 training to keep generating chances at excellent heroes. If you don’t, you may reach a point where you lack heroes of a certain color to level with your feeders of that color.
Do you feed heroes strictly on-color? If not, that will maximize your gains per feeder hero.
Check these threads: