Rules and rebels; guidelines and grouches
It is perfectly reasonable to learn and follow established "rules" of proportion when tying classic married-wing salmon flies. They are a fairly standardized form of craft, well into their second century. Rules make it much easier to learn to tie flies of this artistic school; without them, one could try and err for a long time.
Which has nothing to do with the separate issue of whether classicly proportioned flies are the best of all possible salmon-steelhead flies.
It's the same with architecture: If you want to design buildings in the classical Greek style, you follow certain ratios that were worked out several thousand years ago. That has nothing to do with any issue of whether Greek classicism is the acme of architectural design or not.
To bring it up to date: what are the proper proportions for a tandem-hook fur leech? Five or ten years ago, the question couldn't have been asked. Now, the fly tying community is in the process of refining the strip leech design. In time, there will be a "rule" about this. For now, we're free to experiment, to make leeches as long as water snakes, if we please.