Of course they should, but that doesn't mean they will. We can't reallyforce users to do anything, just like we can't force users to capitalize their "i" and type the full word "please" rather than "plz." I find myself often hitting up Google to figure out what on Earth someone was saying because of unfamiliar acronyms.
You should always assume that the person on the other end isn't familiar with the acronym you're using. Try to expand them as often as possible in your answers. Remember, this is a global audience. Just because all your friends you text with where you live know what it means doesn't mean someone half-way across the world does.
In the end, if you see an acronym somewhere that you think will confuse someone, feel free to edit it to what it means to make the post clearer (of course you can't edit them in comments). As usual, make sure you're fixing up any other issues with the post in the process.
The only real exception I can think of for this is acronyms specific to a certain technology that a question is based on, but it'd have to be something so common that you couldn't possibly not know about it. Even then, I'd much prefer users fully type it out once and then abbreviate it in the rest of the post, to be as clear as possible.