I see "know your worth" advice pretty frequently, and I tend to think it is pretty bad advice for software developers. "Your worth" is completely dependent on the business the employer is in, and then of course a lot of that "worth" can be due to the employer already having a huge market (e.g. I've seen small decisions result in multiple millions of additional revenue, but those multiple millions were only possible because the employer was already pulling in billions).

Maybe I'm splitting hairs, but I think better advice is to simply do a lot of research on what comparable salaries are for the position you're looking for. There are tons of information sources online now that can give good info on accurate salaries, including breakdown of cash, bonus, equity, etc.

To your last point, you better be aware of who has the leverage when you start negotiating. If there are a hundred other qualified candidates, and you really need the job, going into "battle mode" over 2-3 percent is not a good idea. On the flip side, if you really are that "purple squirrel" for which there are few if any compliments, go for all you can get.