In 90% of use-cases and filament types the idler tension can be set to nominal, this includes printing PLA, PETG, ABS, PC, Nylon, PVA, HIPS and others.
If you are printing a particularly hard material, such as carbon fibre reinforced filament, you may need to increase the idler tension.
If you are printing a delicate material of very low hardness, such as colorFabb bronzefill, which are easily indented or even cut through by the fialemnt drive gears then you may need to reduce idler tension.
If you are printing with TPU's, TPE's or other flexible materials you may need to increase the idler tension rather than decreasing it.