While you shouldn't have to baby a prima donna in any department, it does pay to treat people with courtesy and respect. There are tactful ways of giving criticism and there are blunt ways. Some people's manners are downright rude. Treat them as underlings beneath your notice, and they'll spend as little time and effort as possible on your account. If you like the copy, say so.
Most clients only pick up the phone if the news is bad.
But a word of praise now and then can spur a writer to do great things for you. If you like the copy, call and say so. Better still, write a note. Even when you want a rewrite, start off with praise for what you liked about the copy. You may start by thinking otherwise, but you will change your mind.