Senn and etymotic definitely do follow such rules. the matching for left and right is done on the driver for Senn, where they measure them and put them by pairs with the most affinity within a batch, and they simply discard stuff along the way when it isn't to whatever specs they decided upon(but then they make the driver themselves so it's a logical step anyway). I remember someone saying that you could ask Sennheiser for the FR graph of your very own hd800 or hd800s because they not only make those measurements as part of their quality control, they also keep them with the serial number.
for etymotic it's the same idea but they actually deliver what they call a certificate of performance with the latest er4sr and er4xr, but the channel matching care was already a thing with the late er4 series.
both focus on that and tolerate more of the inevitable variations from pair to pair, but try to get left and right in one pair as close as possible with an active selection of the drivers.
now I am positive that I'm not making that stuff up, but I have to admit that I also have a hard time finding it written on the web. for confirmation and information about etymotic it's easy, some cool Ety dudes are active on the dedicated er4 topic.
for Senn, I don't happen to have Axel grell's phone number, mostly because he doesn't have a clue that I even exist, but aside from that we're BFF in my head. ^_^ seriously I'm not sure where to find the information.
https://en-us.sennheiser.com/high-quality-headphones-around-ear-audio-surround-hd-650 here they mention
Systems with narrow tolerances (± 1 dB), hand-picked in pairs
but that's about all I can seem to find right now.
I do have my own measurements for a bunch of IEMs, but it's one pair of each measured by a noob on cheap gears. my pair of er4sr shows better matching than any other IEM I've measured but I wouldn't call that conclusive.
edit: found this