For 4* and 5* heroes, I completely agree. For 3* heroes – not so much. Even using matched-color training and no Trainers, it’s not unlikely to end up with the special not maxed. Of course at that point you can just keep feeding it, but that always seems wasteful of resources.
The highest odds come from feeding your 3* only 1* feeders until the special is maxed. So run TC1 to get only 1* heroes, then shift to TC2/4.