I think it depends on the slang word/phrase and where you are from. For example, I'm from Texas and "y'all" is a part of my everyday vocabulary. [y'all: you all] I don't use any of the ones you mentioned, so maybe you can just learn them as you hear them? If your friends use a slang word/phrase, you can learn what it means then! I hope this helps ^^

They're not as common as people think. YOU DO NOT NEED TO USE ANY OF THEM YOURSELF. They will not make it easier for people to understand. Don't work hard on learning them unless you find them entertaining or amusing. When you hear them in real life you will usually be able to guess the meaning from context. "I'm happy as a clam", coupled with a tone of voice that shows it isn't ironic, obviously means "I'm happy." They tend to be regional, or used within families, or personal. For example, if you are talking about someone whose name you can't remember, some people say "what's-his-name," some say "what's-his-face," and my wife's mom would say "Mr." (or Mrs.) "Tootensnacker." I recently gave an acquaintance a gift, and in his thank-you note he said he was "over the moon," so I've really heard that one. Once. My wife would use the phrase "walking on air." My cousin would say she was "in seventh heaven." But it's hardly necessary to learn before you hear them. If someone writes you a note and says "My son was so excited to get a real baseball glove, he was just ______________ " you know that whatever goes in the blank means "he was really happy." Many of the expressions in lists are included because people think they are amusing, or interesting, or colorful. The lists you see in books are often out of date, possibly by decades. Catchphrases come and go. I don't think you should make a point of studying them unless you enjoy it. People love the idea of knowing things their teacher doesn't know, of knowing the "real" English, and they think learning slang and colloquialisms will put them ahead of other learners. It probably won't.

P.S. Television sitcoms are apt to exaggerate the use of rarely-used catchphrase and idioms because they are colorful and amusing. I am trying to remember which actor was famous for saying things like "as nervous as a long-tailed cat in a room full of rocking chairs." (Of course: it was what's-his-name!)

Slang isn't all that common, except in specific regions of English-speaking countries. Let's be clear here though, slang is generally not something universally accepted. It can differ city to city in some places and doesn't even make sense to other native English speakers much of the time. However, there are still many instances of idioms ("my neck of the woods," among your given examples) that are everyday occurrences, and just as many alternate words in use. In fact, I'd say it can be a serious issue at times when so many different alternate words are used to describe the same thing, even within a single country. There can be a very large number of differences when listening to Australian, British, and American English.

Slang is normal in English speech in terms of shortened words such as "giz" instead of "give me", "'t" instead of "the", "sen" instead of "self", "right" instead of "really" etc