Go with what feels natural. On most trails there is not a perfect place to plant your pole with every single step. Learn to adapt.

I have used poles for hundreds of miles uphill, level and downhill. They can help anywhere if you use them properly. Just make sure they're pushing you up/forward and are not working against you. That being said, I would not use them so much as to be dependent on them.
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If you only travel on sunny days you will never reach your destination.*

* May not apply at certain latitudes in Canada and elsewhere.