I don't know whether my faucets should use flat or beveled washers.
The house was built in1979. I think that's the biggest clue.
Sorry, I don't know who made the faucets.
In one bathroom silnk and tub, there appear to be beveled washers, but that might be because I replaced flat ones before I beieved that flat ones could work. I know I replaced the hot faucet washers in each, and I might have replaced the cold too.
I could check the washers in one of the two other bathroom sinks, or the other shower, but they're not even leaking, and I hate to cause trouble. (One of my favorite phrases!) For example, the cold tub washer wasn't even leaking, but I took out the stem in order to see if the washer was flat or beveled, and what happened? The washer stayed in the faucet! The stub of the washer screw was still there in the stem**, but when I tried to unscrew it, it broke off. Went to HD and they don't have it. Maybe Lowes will or a plumbing supply store surely will.)
**If there were better light, I might have noticed that the washer screw had no head and no slot. I did notice that there was no washer, but I sort of ignored that until the screw stub broke off.