Me personally....I'm from the school of try many boots on that are targeted for your level, your ski skills and the terrain you like to ski.
Some important factors:
The shell should have the same anatomical shape as your foot? (or close to with some grinding)
The flex pattern should be one that allows you to be in a comfortable stance where you work the skis fore and aft with as little movement or as much movement as your ski skills dictate.
Alignment...startin g with the ankle being neutral in the boot...an easy fix with a good footbed but again only if your looking for a change in performace...it may be good for you or may really throw you off.
Knee alignment and tracking...because when we ski, our feet move in parallel with each other no matter how far they angle out when walking or sitting on the chair lift, in my mind, one receives maximum performance if all things in the bodies kinetic chain...from the feet up to the hips are aligned....well at least from my experience. Being able to pressure the outside and inside edges of your feet equally with as little movement as possible I think requires less energy.
Having said all this based on my personal experience...I've skied with many good skiers who "make allowances" for their personal alignment issues and the do it well and they DON"T want to change a thing.
That's why finding a boot that fits you not only physically but mentally as well.
So match your technique and your physical attributes to the way you ski and life will be good. Changing the kinetic chain if you're looking on changing your ski technique works for some but not others....BUT...Upg rading old and worn out boots is the best way to better skiing without a doubt, but matching them up to the MENTAL, PHYSICAL and TECHNICAL skills you are working with is KEY!
Best,
Gary