To that I say pfui! Poppycock!! True, many times when nymph fishing you do want your fly on the bottom, but there are times when you want it off the bottom, too. The trick with nymphs - or any sub-surface fly - is to get it to the level where the trout are feeding.
In part I of this article we discussed adding weight to your nymph patterns with wire, right on the hook underneath the fur & feathers of the pattern. That is a very effective way to get your nymphs lower in the water, but there are other ways to add weight & get more control over just how deep in the water column your fly will be drifting. It takes some thought & experience to get it right.
For those of you who have ever watched various mayfly nymphs or caddis larva move across a stream bottom, you've probably noticed that they don't always crawl. Most of the time, especially if there's any kind of water current, they tend to bounce their way down stream. They move along the bottom for a little bit & then are picked up by the current (some actually undulate or swim), lifted a few inches or so, & then drift back down to the bottom. This repeats over & over as they work their way down stream. Often trout will take them on this bounce when they're off the bottom. It's easy pickings for the trout.
You want your fly to imitate that movement of the natural. One good way to do that is to add weight to your leader. I like to use round split-shot in size BB, but there are other types of on-leader types of weights. Use whatever type you like.
You want to add enough so that it pulls your leader, or tippet, down to the bottom. You then adjust how far up from the fly to put the weight(s). You see your nymph, being lighter than the weight on your leader will float above the weight that is on the stream bed. The water current pulls the fly down stream, which also drags the weight along, too. As they both move down stream your nymph begins to sink towards the bottom & then move back up again as your weight starts to settle back down on the bottom. All this over & over again as your nymph undulates, rises & falls, as it drifts down stream.
***The farther you put your weight from your fly, the higher it will "bounce" off the bottom. You need to know how far off the bottom the trout are taking the naturals so you can get your fly at that same level. Likewise, the closer you put the weight to your fly, the closer it will drift to the bottom. Keep experimenting with the weight closer or farther from your fly on the leader until you start picking up fish.
Many nymphs continue this up & down motion on their way to the surface to hatch. They keep going higher & higher as they continue to rise & dip. They are slowly working their way to the surface to hatch. Again, experiment with how deep you fish your flies. You might be surprised to be taking trout on nymphs just below the surface in the minutes before a hatch begins.
My entire point here is that you should always try fishing your nymphs at different levels, or depths. Also beware of anybody telling you there is only one "correct" way to fish a fly. You should run from well-meaning, but crippling, dogma. It helps a ton to be open mined & a little creative in fly fishing.