"The truth hurts", "ignorance is bliss", and "what you don't know can't hurt you" don't contradict: they all say you're better off not knowing some bit of information that would be unpleasant to know, or that a small "white lie" is allowable.
The opposite there would be phrases I've mostly seen via LessWrong like "that which can be destroyed by the truth, should be", or "what is true is already true, owning up to it doesn't make it worse", or "if you tell one lie, the truth is thereafter your enemy", or the general ethos that knowing true information enables effective action.
Or to add on to the thought, there are non-LW pro-truth/knowledge idioms like "knowledge is power", "the truth will set you free", or "honesty is the best policy"
“Don’t judge a book by its cover” or “No pain, no gain.” Others have an opposite proverb that’s similarly common and reasonable.
"If it looks like a duck and quacks like a duck" is likely an opposite for the first, and "The best things in life are free" is at least plausibly counter to the ladder.
Some proverbs are actually autoantonyms, or at least have come to mean the opposite of the original intent. For example, "Blood is thicker than water" nowadays means that the deepest bonds we form are family-related-by-blood. But I've read that originally the saying was "The blood of the covenant is thicker than the water of the womb," aka Christians should feel deeper bonds to other Christians than to their own siblings in cases where the two conflict.
It's a dog-eat-dog world.
This one's better in Latin: Homo homini lupus. Man is a wolf to man.
The people cry out for a strong leader.
This one's better in German.
My favorite adage, from Schiller, is "… the gods themselves contend in vain."
"Two heads are better than one" and "Too many cooks in the kitchen ruin the broth" also aren't necessarily opposite. It reminds me of the phrase "two's company, three's a crowd". It's possible for two to be better than one while three (or an even larger number) is too many. Another fun saying along these lines is "A man with a watch knows what time it is. A man with two watches is never sure." which seems to usually be used to mean that if you have only one source of information you can become overconfident, whereas if you have two then you won't know which is correct if they contradict, and even if they don't contradict you are more likely to be aware of the fact that they could be wrong. Another one is "a camel is a horse designed by committee"
I can only remember times I've heard "Better safe than sorry" used to encourage taking an action that had a high chance of being a small waste of time or effort, with a small chance of minimizing the consequences of an unlikely event that would make it worth it, such as taking an umbrella in case it rains or seeing a doctor in case there is something wrong. I can similarly only remember "Nothing ventured, nothing gained" used as a way to encourage taking actions specifically usually also ones that have a low chance of a high reward, and a high chance of a small loss. These do not seem like opposites to me.
Some proverbs are actively suspicious, like “Don’t judge a book by its cover” or “No pain, no gain.” Others have an opposite proverb that’s similarly common and reasonable.
But the four below I use often: