A lot depends on the juice you are using, most juices have a certain heat they like depending on your tastes as well. The lower the resistance the warmer they get on the same voltage. Some juices give a better flavour at higher heat then others.
My main resistance used is 2.1ohms but I have some juices that I use 1.8ohms for and others I use 2.5ohms for using the same voltage. Once I find the resistance for each juice, I then just tweak the voltage up or down slightly to to find my perfect sweet spot for each one.