In general, the smaller the fly, the less "accurate" it has to be. The fish has very little to see "wrong" with it. Larger bugs and their imitations are scrutinized easier.
This is why an Al's trico works so well, but the same fly tied as a green drake would never get a look.
Having said that, there have been many instances that an Al's trico was refused several times and the same fish has eaten a more accurate imitation on the first cast. This has happened almost exclusively on fish that have been caught a few times, are larger than average and see a whole lot of flies in a season.
For me, I tie dry flies without tails only 24 and smaller, but also have more accurate imitations as well.
I haven't done this on size 20's, so I can't comment.