Dom Swentosky, over at Troutbitten, penned this story on a topic that I’ve long since believed in – that you don’t always need to match the hatch.
Often, matching the hatch exactly results in your fly being lose among the dozens of naturals. Un-matching the hatch, though, makes your fly stand out. So long as you’re not trying to fish a salmon fly in the middle of a trico hatch, you should be good to fish a slightly different bug than what you see on the water.
Dom goes into more great detail, which you can read in full here.