It is hard to setup a general guideline for matters like these, however when this is transport calculation, then most interesting things happen around the fermi level.
Therefore the energy range should always included the zero energy, and therefore I think that renren12313 suggestion about the energy window should be from -2 ev to +2 eV is good. I usally do it from -5 eV to 5 eV just to get all the details of the spectrum, even if the range from 2-5 does not provided alot of information.
Regarding the enery spacing (aka the resolution) it depends on the system you are working with. If you are studying the transmission in ideal systems, the resolution could be quite low (~0.1 eV), however if the systems involves both spin-polarized metals and high band-gap semiconductors interfaces etc, then you should go with a better resolution and use something like ~0.01-0.05 eV. But again it is question of what kind of details you want, since increasing the resolution (lowering the energy spacing) will also make the calculation more time consuming.
So for me it always boils down to: Do I want to see the trend (higher energy spacing) or do I want to see the details? (low energy spacing)
For example I have posted a series of transmission spectrums on
strain of atomic wires ,
where the goal was to understand the what happend when a wire go strained more and more. Therefore I went for the trend and used a higher energy resolution.
