The famous nocturnal Hex hatch of the Midwest (and a few other lucky locations) stirs to the surface mythically large brown trout that only touch streamers for the rest of the year.
A guide friend of mine likes to say "cameras kill trout".
On the Missouri, which I know intimately and have caught hundreds of 18" - 22" browns and rainbows (and a handful of 23" - 24")I usually never go above, or below, 5X flourocarbon. Once in a blue moon for a particularly difficult fish or fish that are very leader shy I will go to a piece of 6X about 36" long. If one plays his fish effectively and is not showboating most any river trout can be in the net in under eight minutes.