i think you can get away with resistors when you can work with 5V, up to 1.5A. lithium batteries need current limiter in series with voltage limiter, and this alone can be made in a very simple way, unless you want to know when battery is charged, or if you want extra efficiency that smps gives you, but this only makes sense for larger batteries (phone-sized and up) or when you want to handle everything to charger and connect battery to charger directly. then i think you need controller
i think you can get away with resistors when you can work with 5V, up to 1.5A. lithium batteries need current limiter in series with voltage limiter, and this alone can be made in a very simple way, unless you want to know when battery is charged, or if you want extra efficiency that smps gives you, but this only makes sense for larger batteries (phone-sized and up) or when you want to handle everything to charger and connect battery to charger directly. then i think you need controller