I can get by just fine running with variable voltage, in fact, I prefer it. Of course, my mods include a wattmeter and an ammeter. I actually find current a more useful gauge than anything else, believe it or not. You can get the most consistency between various coil lengths by running the same current through each one. Makes sense since coil wire temperature is going to be dependant on current. Power only gives you the total heat, not the actual wire temperature. A long coil with higher power can actually have lower wire temperature than a short coil with lower power even though total heat is greater with the higher wattage. It's the temperature the juice sees when it comes in contact with the coil that can make a big difference.
Generally speaking, there are just too many variables for power to be the end all gauge in tuning atomizer performance.