Mostly this topic is so I can stop typing in numbers and just quote it for other posts.
It depends on your playstyle.
Gryphonkit, my wife, uses RT1- RT20 including single color training.
See also this excellent post on the recruit bank.
However, there is no limit on holding finished heroes in extra low cost training. I had over 2500 finished 1*/ 2* when I pulled green 4* Hansel, so I picked four other heroes, same color leveled them all to 4* 3.60 in less than 24 hours and still had approximately 1300 finished 1*/ 2* heroes left in camps ( didn’t write the number down ).
I used RT19 until I realized it only gave 1* Heroes per 2 recruits. Even at 2 recruits per hero, that is much less hero XP than RT11. RT11 is slow, but you can make up for it by running three camps at RT11 and there is no need for any ascension items.
MY PERSONAL OPINION
Devs could change RT19 to 1 recruit per finished 1* Hero if they wanted to make it more attractive. And have single color training give you 1*/ 2*/ 3* heroes at 2.1 instead of 1.1. But there is little monetary incentive since the current system encourages wealthier players to use 3* heroes from summons to feed their 4*/ 5* heroes.