The amount of attenuation you need is situation dependent. If you have just a little noise floor hiss or just need a little more volume control then 12dB might be appropriate. If you need more then 18 and if you're using a 100+ watt amp then 25dB or more might be warranted. For most people going with 18dB to start with is probably best.
In other words put it on the amp's input. These attenuators can make your amp's noise floor hiss worse because your signal is being pushed down toward the floor (which is not lowered) where an attenuator on the amp's output pushes everything down including the noise floor.