If you're tying specific patterns, the recipe should describe the type of hackle used in the original. Otherwise I would say it depends on the pattern style and which part of the fly (collar, wing, tail etc.).
For the muddy estuaries of Costa Rica, patterns that push a lot of water and kick out a bunch of vibes would call for a good stiff cock neck whereas a clear water palmered worm pattern would have a lot more life tied with a softer hackle.
Saddlles are great for palmering on long shank hooks. I guess hen could have an application also.
When sight fishing, look over your shoulder from time to time, you never know who's behind you