Dear Miss Snark:
I know it's never okay to badmouth a publisher, but...is it ever okay?
Seriously. What if this is a legitimate complaint about unpaid royalties, or the "buy" link for an ebook not working for weeks on end? If you plan to complain in a professional manner, not "Publisher X stinks" posted all over your website and blog, but just saying that you've had problems with them, and these are the problems, and not everyone's experience might be that way but yours is?
What if the process to break a contract is rather confusing and involves copies of letters sent to people whose names are not given anywhere, but if you go public with your complaint there's a good chance they'll drop you?
What if you already have a different publisher? Does any of it matter?
Is it ever okay to voice real discontent? Or would I be a nitwit no matter what the situation is?
The key components are adjectives, and first person. If you are stating facts, you aren't badmouthing. Saying the publisher is a dirty rotten cheating scoundrel and anyone who does business with him/her/it deserves what they get is a far cry from "they didn't pay my royalties on the agreed upon schedule". Using your own experience to state facts is important. That means "and I've also heard" is not ok.
And yes, it's ENTIRELY ok to make facts known in a calm, businesslike manner. Sharp operators depend on people not hearing about their inadequacies until it's too late.
Badmouthing is emotion laden ranting along the lines of "they didn't do enough for me" or "they didn't ever want to talk to me on the phone". You don't know what they "wanted" you only know your calls weren't answered (and there are plenty of people who think I never answer the phone either since I insist they email me because they're such pains in the ass I want written records of all conversations).
You want to keep this to a minimum of course but you're not going to shoot yourself in the foot to discuss your experiences in a calm rational way. Leave the flaming coiffures to those of us with fire extinguishers at the ready.