I actually do this on some of my flies but not by actually physically "flattening" the bodies after tying and I only flatten the bigger flies that you can take advantage of the shape and where it is a very noticeable part of the anatomy like in the Stenonema nymphs namely the March Browns and Gray foxes (and other cahill types) with their exaggerated flat profile and gills.
This is one of 2 different March Brown nymphs I tie which uses flattening the profile by the addition and then reduction of material to trim it into a flat shape. Overdub , pick, rib, pick, trim to shape. Keep the body from thread head to base of tails about 15-16mm on the March Browns.
I do this for the MB, GF, cahills and even a little on quill Gordon nymphs.(but obviously alter the color and sizes accordingly)
This is the second style I tie. Overall similar construction and hook/body length but a little more elongated on the wingcase for realism, and I'll tie it a little whispier in the abdomen and add dyed grizzly for the tails and a small tuft for gills along with the thinner picked gills for a bit of soft movement as well.
The other clingers like Hendricksons and Sulphurs or PMDs I tie more robust and go for the overall bulk of the nymph as if it could be seen in any direction as the smaller nymph tumbles through pockets etc. Not QUITE what you'd call in the round because they are still anatomically correct I just don't think the "flat" component of their shape is as important as those super flat wide MB's etc. and the trout seem to like them just fine as is.
Here's an example of the smaller Hendrickson nymph, I'll even work in the amber banding on the abdomen but I don't make any effort to flatten beyond giving it a little squeeze with the fingers to square the wingpad and remove any high spots.
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