It is always the customers version of covered that counts, they are the ones that must be happy. If you bid two coats and the old color shows thru arent you going to make them happy and apply another coat? I simply propose to "cover" so as to avoid arguments over how many coats are on the surface. I always inspect my work before I present it as finished to my customers. You only need one bad reference to hurt your reputation in a small town. This is what I have "learned".
I take your word for it and that makes you a minority. As an experienced painter you generally know when something needs two or three coats right? Why not specify and give a more accurate quote? That way you don't screw the client OR yourself. Just my opinion.