Having learned to play the drums "the hard way" (i.e. autodidact + electronic kit first; acoustic only really after a few years), I urge you to figure out the volume differences between instruments on an acoustic kit first, especially between different instrument families (snare, bass, toms, crash, ride, hats, etc).
Try to setup your electronic instruments to match their acoustic counterparts as closely as possible. That way you will get a much greater sense of playing dynamics (relation between your movements and the volume they create) in the long run.
This is of special importance if you spend most (I'd say more than half) of your time playing an electric kit.
Come to think of it, this whole dynamics thing was actually the hardest part of re-learning drums on an acoustic kit.
Considering the audio mix only, I'd go for different sound characteristics over volumes.
All modern e-drums worth their value have a decent sound library to pick from for every instrument. Leveraging that, you can achieve a clean mix with little to no volume differences (though they still should be considered, if only for above reasons).
Look for highly distinguishable sounds for every instrument family. It won't matter if all your crash cymbals have the same overtones, or your toms have the same type of shell (they all differ in size and, thus, pitch). Just make sure snare really sets itself apart from toms as well as your ride ain't just a larger crash.
For example, Roland's V-Drums classify their sound samples by materials or musical styles.
Pick some wildly different sounds initially and work your way from there.