Bass are ADD. They could care less about the leader. They see bug, bug means food, food means kill! You just have to get it in front of one. For a bass leader I will use anything that will turn over the bug I'm tossing. I don't get picky. Big bug means heavier mono and conversely small bug with lighter leader. I don't even worry about taper with bass.
For trout, completely different story. Again you need the proper leader to turn over the bug of choice but there are a lot of other factors that go into spooky trout fishing. Usually longer finer leaders required here. Trout can be really leader shy if given the chance to see it. Fish appropriately.
I have been thinking about how to reply to this thread, and I have not read anything above I disagree with. So keeping to DH rods and floating lines, here are my thoughts
1. You need a leader to help you set-up, which probably means 9' minimum, but if you can go short er, then do so. You will not affect fish catching in most cases - especially for bass.
2. If I am going to fish small wets and spiders for trout, I would think 9-12'.
3. Leaders do not need to be complex, and I prefer the stiffness of Chameleon at 30# at tip. I trade a smaller diameter leader top for stiffer material.
4. In the last 3-4', I transition to fluorocarbon - 10-14lb Seaquar Blue. If fishing a wee wet, then I will go to 2 steps in fluoro to 6'.
Right now, my bass want to hit the fly (streamer muddler) below the surface. It does not need to be 3' below, but it must be swim below. Place the fly on the surface will tempt them, but they are cautious on those last 6" to the surface. Maybe, it is the Osprey or Bald Eagles, but they get cautious in those last inches.
Then I use a 3D Scandi line from Guideline, but I am using a rod with a higher grain window from Meiser. Wither a FHI or FIS2
I have sink tips also with my Meiser 1264 - 324gr line and Steve included 10' RIO