That really depends on what you want to simulate. These metallic and dielectric regions are "real" parts of the system, so your question is a bit like "should I simulate a C60 or C80 fullerene in the middle?" It depends on what experimental situations you are looking to match, or what effects you are looking for. But generally speaking, if you are aiming to "bias the molecule", it makes more sense to have a localized gate, with the understanding that you are actually trying to create a "gate" that is just a few atoms wide - it's unclear how realistic that is... The dielectric region in turn acts like the whitebox on a camera when using a flash, it softens the effect of the field, but whether or not it is necessary or relevant again depends on whether you can motivate it from the model perspective.
