In the late 1960's when I really started fishing with a fly rod, my standard tippet was 3X. 3X!! Not because I was fishing for tarpon with a dry fly, but because it was three pound test. I would break off too many fish if I used 4X and 5X was out of the question. 3X, three pound test was the best technology had to offer. Luckily, I was mostly fishing #12 Royal Wulff's in the Montana backcountry. When I look at the diameter of that 3X leader today, I am really thankful for the modern 5, 6, and even 7X that is so much stronger. I know for certain that in many trout waters tippet size does make a difference.
Only yesterday, on a small Colorado Front Range stream, I tried a #10 hopper pattern on a 4X leader. No rises, not even a looker. All of the fish in this stream were wild brown trout in a fairly remote area and don't have to duck fish hooks on a regular basis. I extended my leader with a two foot section of 6X and recast. A trout rose on the first float over the same water. Fly size or pattern was not the factor this day, but tippet size was.
Can fish really see a difference between 4X and 6X? They may, but that is not the only factor. The lighter leader aids in presentation.
Did you hear the one about the two guys in a boat? Both used the same rod, reel, fly, etc., but one caught 10 fish and the other caught one. Maybe you have been there. What could be the difference? Presentation. If the fly lands with a splash, the line crashes into the water or the angler rips a "bad cast" off the surface leaving a spray; these all add up to poor presentation. Trout, wild or hold over stockers, learn quickly that things moving through the air over their heads may mean danger. Freshly stocked fish may be attracted to the sound of pellets hitting the water, but after surviving a month or so in the river they may learn that a plunk or a splat is not necessarily related to food, but to disaster.
If you have heard that 10% of the fishermen catch 90% of the fish and are determined to join the elite 10%, then consider the size of your tippet and the length of your leader. Lighter and longer may sound a little scary to beginners or even old timers not used to it, but it does make a difference. If you use a 5X or a 6X tippet, on a leader that is 1 ½ times your rod length, you will get more strikes. You may not land more fish right a way, if you are not used to the lighter leader material, but with practice you will wonder why those other anglers are using the heavy stuff.
The other aspect of delicate presentation involves using a softer rod. Pool cue stiff fly rods tend to cause a larger splash down area when the fly, leader and line land. Four and five weight rods with mid-flex action create softer landings and fewer disturbances. Some rod companies say that this action is "traditional" meaning, I think, that it compares to fiberglass or bamboo rather than graphite. All graphite rods are not poker stiff and certain rod models by Temple Fork, Powell and Orvis have action recommended for meadow streams, spring creeks and quiet waters. These are not parking lot rods, designed to cast entire fly lines, but rods to be used at medium distances of 30 feet or so. Their function is to put a dry fly or nymph in a tea cup with only a minimum of disturbance. Long rods, 9 feet or more provide better line control, easier mending and less drag. All this adds up to less disturbance, better presentation, more strikes, and more fish…the elite 10%. Be stealthy, be watchful and be creative to catch more fish.
Dennis Galyardt