This thread needs more barakta. She has snot stories that would give anyone who isn't an ENT regular nightmares.
My default state is breathing through my nose. That's what it's for, and enables you to smell things and produce London-coloured bogies and stuff. I switch to mouth breathing when higher flow is required during exercise in the normal way.
Except for all the times when my nose doesn't work, and I'm forced to breathe through my mouth. As a chronic hayfever sufferer this is quite often. I also have a related failure mode when my nose is partially congested, to the point where enough flow can be maintained for normal breathing, but the resulting rise in pressure through the
crustacean eustachian tubes causes pain/itching/loud clicking sounds from one or both eardrums. That's particularly annoying at night, as I'll tend to hold my breath rather than mouth breathing to make it stop.