If the hatch that is feeding the fish is small then your 18's, 20's and even 22's will be an important part of the game. You can offset some of those hatches some of the time with slightly larger in a matching pattern, say a 14 when you see 16's or by using some other stimulator or attractor pattern.
I spent some quality time with a fine flyfishing gentleman on a very well known and respected westslope cutthorat trout. He came to the fishing outing with a full box EACH of 14's, 16's and 18's and a few other assorted patterns. To say that he impressed me in knowing what he was doing, how to do it and had his game dialed in just right would be an understatement. I learned a lot that trip, and some of what I learned was how critical a fly pattern AND pattern size can be.
A slow action fly rod would be helpful with flies that small and the matching tippet. A large and aggressive fish would easily make short work of your tackle if your rod wasn't much of a shock absorber.
Oh, the largest rainbow I ever caught at Pass Lake was on an olive #18 beadhead hare's ear nymph.
Absolutely they are worth it. in fact sometimes thats the only thing that will work. For trout selectively feeding on a certain bug, at a certain stage in it's life cycle, you must "match the hatch" very well or the'll give you the "fin". If the bugs are a size #20, you will need to use that size for the nymph, as well as the emerger, and adult. Of course there are times when you can fudge a bit and get away with it, but a lot of times you've got to get it right. I use as heavy a tippet as I can get away with, usually 5X down to a #18 or even #20 hook, after that I'll drop to 6X, and as mentioned above, a medium action rod will make a huge difference in tippet protection. Fast rods are great for distance, but they suck for protecting small tippets and big fish.
If you are worried about getting a bigger tippet, 4x, into them small eyes here is a tip. Snip off your tippet on the bias and thread the sharp end through the eyes. I use Zebra Nymphs as small as size 22 in the winter time here in Montana. Also a steady hand and good eyes are also helpful.
Those little flies are very important in rivers like the South Platte, where at some times of the year the trout won't even look at sizes larger than 20. When I was there in March fishing the 11 mile canyon and the Cheesman canyon, we were fishing size 22, 24, and 26 emergers. That was they live on down there. As far as tippet goes I guess it just depends on how pressured the fish are. My profile pic fish was caught in the 11 mile canyon on a size 24 cheesman emerger on 8x.
I have a simple formula. The line in the sand has been drawn at size 18 and 6X tippit. If fish are such snotty prima donas that they won't eat an 18 or larger then I will just change venues. There are millions of fish that will happily eat an 18 or larger flies and those are the ones I want to become acquainted with.
Using those micro flies and gossamer leaders is a form of self punishment that I would equate with dating a high maintenance broad. Sure, you can do it but if you haven't figured out what the end result is in pain and grief then you are probably doomed to endure it.
C'mon Steve-what is this "old" crap? You are about half of my age and you claim to be blind? If you can't tell a buck from duck at 100 yards maybe you better think about forfeiting that hunting license.......Mr.Magoo and a blunderbuss=not good!