I'm lazy. I test things with the flux still on. No idea if it is meant to be no clean or not, lol. I haven't fried anything though. Are you suggesting tin whiskers, perhaps? I wouldn't think you would have much to worry about on keyboard pcbs, where most of the contacts/pads are pretty far away from each other.
My understanding is that in general for many things a bit of flux isn't going to cause an issue, especially things like keyboards that aren't operating at high frequencies and whatnot. The main issue is that flux is often acidic to some extent, and depending on the PCB/components/flux/conditions what can happen is flux that isn't cleaned can cause corrosion and failure later on down the road. The most common sort of flux in hobby type solders is rosin. This is fairly mild stuff and often doesn't cause issues down the road if not cleaned. Other types of flux can almost certain cause issues later if not cleaned off. Best practice is to always clean flux off after soldering, but I've soldered a number of things with rosin core solder, not cleaned them, and they're still functioning years later.
In a lot of commercial settings, my understanding is they typically use water soluble flux (rosin flux is
not water soluble). They then run the soldered boards through a PCB washer. Essentially this is a glorified dishwasher that usually washes them with deionized water (alternatively these machines could use chemicals for non-water soluable flux). In fact, some components, such as buzzers, actually
come from the manufacture with stickers over the hole. The purpose of these stickers is to keep water or cleaning chemicals out of the buzzer when it goes through a board washer after soldering.
TL;DR: For hobbyists, not cleaning flux
usually isn't a problem, but in industry it is usually always done.
