Generally, yes. But some guides or tutorials don't respect this convention and use any number of chars including the author's own personal prompt. (I see `>` used often too).

I would always use '>' in README files to indicate the command was to be ran in a command prompt window for Windows users, usually followed with context.

I'm guilty of this, as I have my prompt set to a single > character. I'd be interested to know where the $ as a representation for user input suggestion originated from. I didn't know about # indicating that a command should be run as a superuser

> I'd be interested to know where the $ as a representation for user input suggestion originated from.

In the absence of any configuration, the default prompt for most shells is (or was) simply $. When you su to root, it becomes #

Indeed.