If you all really want to know, the flex rating of your boot (should) comes down more to your flexibility and biomechanics than your intended use. Dorsiflexion, the amount you're able to bend your ankle, varies between people quite alot.
Low dorsiflexion, example me, means you have lower flexibility, in which case you can't bend your ankle huge amounts, and typically you favour higher flex boots (my standard flex is 120, and I'm not huge, 80kg, 176cm). A higher flex means you're supported by the boot through your full range of motion, and the boot will react better to changes in pressure in a limited range of motion. For someone with low dorsiflexion in an overly soft boot, they will get very tired, and feel like the ski is not reacting as well to small changes in pressure.
High dorsiflexion is the opposite, lots of ankle motion. Typically, due to day to day movements, these people have a tendency to be very mobile with their body, changing the centre of gravity very quickly and easily. If they skied in a high flexing boot, they would be bucked around alot as the boot doesn't flex with the body movements. Softer boots are more forgiving and therefore easier to ski for these people.
From here you can make adjustments based on preference. You might opt for a softer boot if you have low dorsiflexion if you're after a more forgiving boot for more comfort, and don't mind the fatigue and ski with a more centered stance without flexing the boot too much. If you have high dorsiflexion you might go for a stiffer boot if you're more advanced and know how to move your weight to avoid being bucked around etc. The purpose of the boot also comes in - freeriding and racing typically want more responsiveness, which is where the higher flexes come in, and casual skiers looking for comfort might go softer to avoid shin pain or allow them to ski less athletically.