Tvan,
It is the gauge of the wire, the temp your juice starts to burn, the temperature your wicks and coils start to plug/coat up at etc...
Some atomizers come with recommended voltage charts like Kanger tanks. But even with the Kanger I usually run above their recommended settings. I have just spent the last two days running at their highest recommended value and found not only did those setting make the juice taste really bad but it actually made me a little sick to my stomach. After reading this thread a couple of times I decided to go about two watts higher than the Kanger recommend settings and all is good once again.
My personal opinion is do what you have to do to make the vape and the taste right for you. The price you might have to pay will be the need to replace your atomizer coils and wicks a little more often. Most other excesses will be taken care of by the smart battery and/or the mod safety circuits.
I tend to get a little too technical with my toys and all it ever seems to accomplish is take all the fun and pleasure out of vaping so I try hard not to let my OCD take control of my vaping experience.
One caveat: If you are using a completely dumb battery in a completely dumb mechanical mod I would at least stick to the middle zone of one of these chart recommendations. I have grown a little fond of ten digits on my hands besides they are part of my poor man's calculator.
PS - you don't need to learn Ohm's law this will calculate it for you and you won't even have to use your fingers:
Ohm's Law Calculator